# Are objects in the universe moving away from each other at the same acceleration?

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This is a follow up question to Does the universe expand at the same rate everywhere in the universe?.

If the universe is expanding, then it would seem necessary for each object (ie galaxy) to be accelerating away from every other object at the same rate of acceleration which is constant (or not?). If that is true, what is the acceleration?

In other words, relative to the Milky Way, does it seem that all the other galaxies are moving directly away from us at the same rate of acceleration? If so, is it possible for us to prove that for any other galaxy that we know, the same observation would be true, were we able to view the universe from that galaxy?

I assume you mean is $frac{dv}{dt}$ the same for all objects and/or is it constant for all objects, where $v$ is recessional velocity (and $t$ is cosmic time)? The answer is no it is not the same for all objects and it is not constant for all objects.

$$v=frac{dD}{dt}=H(t)D$$

Where $D$ is proper distance and $H(t)$ is the Hubble parameter. If we take the Hubble parameter as a constant, then:

$$D = D_0e^{Ht}$$

Where $D_0$ is the proper distance at the present time. So,

$$D"(t) = frac{dv}{dt} = D_0H^2e^{Ht}$$

So the "recessional acceleration" of an object depends on its present proper distance and is exponentially increasing with time.

Now the Hubble parameter isn't a constant and in LCDM cosmology is currently asymptotically decreasing to a constant. In the current epoch though it is fair to say that the "recessional acceleration" of an object depends on its proper distance and is increasing with time.

Here's a non-mathematical answer. Something you can think about in your head. The further away a galaxy is from us the faster it will move away from us. There is no exact way to measure the exact speed because the distance is phenomenal, however, we can use red-shift to work out that this is actually happening. A way to prove that this happens is to get a balloon and draw dots on it. Blow it up a little bit, maybe half way. Focus on one dot and blow it up even more. You will notice that the other dots move away from the one you focus on faster than the dot you are looking at. This is a good way of explaining what is actually happening in the universe - and why things further away from us move away faster.

This applies to whatever galaxy we are in. If we were in the andromeda galaxy, we would still experience every other galaxy moving away from us faster.

## Universal Expansion is Accelerating to Speed of Light?

If I understand rightly what I am being told, We are told the universe is expanding at an increasing rate of speed, i.e. expansion is accelerating. Is it stars moving away from each other within a Galaxy? Or is the expansion a matter of Galaxies moving away from other Galaxies?

At this rate of acceleration, how long a time will it be until stars (or Galaxies) are moving from each other at the speed of light?

Is it the same answer for all stars at all parts of the universe?

### #4 DaveC2042

If I understand rightly what I am being told, We are told the universe is expanding at an increasing rate of speed, i.e. expansion is accelerating. Is it stars moving away from each other within a Galaxy? Or is the expansion a matter of Galaxies moving away from other Galaxies?

At this rate of acceleration, how long a time will it be until stars (or Galaxies) are moving from each other at the speed of light?
Is it the same answer for all stars at all parts of the universe?

The expansion of the universe is not that things are moving away from each other. It is that the space between them is expanding.

Each galaxy is actually at rest.

### #6 Luca Brasi

Galaxies are already accelerating away from us faster than the speed of light. at a far enough distance.

Here is a great video on the cosmic horizon.

### #7 Stellar1

Space itself is expanding, matter cannot achieve the speed of light.

### #8 Keith Rivich

If I understand rightly what I am being told, We are told the universe is expanding at an increasing rate of speed, i.e. expansion is accelerating. Is it stars moving away from each other within a Galaxy? Or is the expansion a matter of Galaxies moving away from other Galaxies?

At this rate of acceleration, how long a time will it be until stars (or Galaxies) are moving from each other at the speed of light?

Is it the same answer for all stars at all parts of the universe?

Brian

Local gravity overwhelms expansion so stars and galaxies will combine and merge despite expansion.

I believe somewhere around 2bly is where we, as visual observers, start to see the effects of expansion. Galaxies and quasars at this distance start to show a color shift due to expansion.

Interesting note is that to an astronomer on some way far off galaxy already sees us "moving away" at near light speeds! For observers that are way way across the universe we are "moving away" at speeds greater then c and we will forever be out of reach of their telescopes. And them ours.

### #9 Brian Albin

Thank you, Keith Rivich. Those points helped me to understand.

### #10 BillP

Not your speed of light question, but since scientists are using the concept of dark energy to explain this increasing expansion, there is a scenario in that model where the increasing dark energy can cause the pull of expansion to be so great that it overcomes gravity, strong and weak forces in atoms, etc. It is call the Big Rip - https://www.universe. is-the-big-rip/.

### #11 Cali

The expansion of the universe is not that things are moving away from each other. It is that the space between them is expanding.

Each galaxy is actually at rest.

(Just pulling your leg DaveC2042)

How are the Southern Skies?

Edited by Cali, 31 May 2019 - 08:00 PM.

### #12 BillP

The expansion of the universe is not that things are moving away from each other. It is that the space between them is expanding.

Each galaxy is actually at rest.

Hmmm. I would not agree as it is a frame or reference issue. If space were to expand at a speed greater than that of light, then from the frame of reference outside of space-time, the galaxies would be moving apart from each other at more than the speed of light, even if they were at rest in space-time. And when and if a time comes that space-time does expand at or faster than the speed of light, then the universe becomes "dark" to all of us observers in the universe. So at that point amateur astronomy will no longer be a hobby as nothing to look at or image. https://arxiv.org/pd. -ph/0107568.pdf

Edited by BillP, 31 May 2019 - 08:09 PM.

### #13 Cali

And when and if a time comes that space-time does expand at or faster than the speed of light, then the universe becomes "dark" to all of us observers in the universe. So at that point amateur astronomy will no longer be a hobby as nothing to look at or image. https://arxiv.org/pd. -ph/0107568.pdf

Well, I guess I better put my scope up for sale on CN.

And here I got into backyard astronomy as a form of escape from everyday existential dread.

Thanks a lot BillP, you party pooper.

Edited by Cali, 31 May 2019 - 09:48 PM.

### #14 Brian Albin

"If space were to expand at a speed greater than that of light, then from the frame of reference outside of space-time, the galaxies would be moving apart from each other at more than the speed of light, even if they were at rest in space-time."

Is this to say there is no space time in the darkness between Galaxies?

### #15 Brian Albin

I mean, I don't think that is what you meant, but is that a valid implication?

### #16 ColoHank

The expansion of the universe is not that things are moving away from each other. It is that the space between them is expanding.

Each galaxy is actually at rest.

I've experienced that phenomenon driving across eastern Colorado and Kansas.

### #17 Mister T

I've experienced that phenomenon driving across eastern Colorado and Kansas.

"Just over this next rise" where the next rise is the curve of the earth

### #18 Pess

If I understand rightly what I am being told, We are told the universe is expanding at an increasing rate of speed, i.e. expansion is accelerating. Is it stars moving away from each other within a Galaxy? Or is the expansion a matter of Galaxies moving away from other Galaxies?

At this rate of acceleration, how long a time will it be until stars (or Galaxies) are moving from each other at the speed of light?

Is it the same answer for all stars at all parts of the universe?

Brian

1. The 'mass' within and without galaxies is not accelerating away from each other. All mass has proper motion influenced by gravity only.

2. What is happening is that the 'space' between everything is expanding or stretching or pouring in or whatever. Think of a large pool of water with two boats next to each other. Now imagine pouring water in between them and, as the water volume increases, the boats separate. Not a perfect analogy but you get the drift (bad pun alert!).

3. As this continues our 'light horizon' expands. Stuff very, very far from us is now expanding away faster than the speed of light. Not because the mass is moving that fast, but because the continuing interspatial expansion over cosmic distances is creating additional distance at a rate faster than the speed of light.

Pesse (See dat? I didn't even use the 'raisins in rising bread' analogy! So proud!) Mist

Edited by Pess, 06 June 2019 - 08:04 AM.

### #19 Keith Rivich

Interesting question.

1. The 'mass' within and without galaxies is not accelerating away from each other. All mass has proper motion influenced by gravity only.

2. What is happening is that the 'space' between everything is expanding or stretching or pouring in or whatever. Think of a large pool of water with two boats next to each other. Now imagine pouring water in between them and, as the water volume increases, the boats separate. Not a perfect analogy but you get the drift (bad pun alert!).

3. As this continues our 'light horizon' expands. Stuff very, very far from us is now expanding away faster than the speed of light. Not because the mass is moving that fast, but because the continuing interspatial expansion over cosmic distances is creating additional distance at a rate faster than the speed of light.

Pesse (See dat? I didn't even use the 'raisins in rising bread' analogy! So proud!) Mist

But you did use the "raisins" analogy, right there in your signature. Kinda like a post script.

### #20 Pess

I've experienced that phenomenon driving across eastern Colorado and Kansas.

Pesse (I think that exact activity is what gives Flat Earth Society members inspiration.) Mist

## The Universe Is Disappearing, And There's Nothing We Can Do To Stop It

After the Big Bang, the Universe was almost perfectly uniform, and full of matter, energy and . [+] radiation in a rapidly expanding state. As time goes on, the Universe not only clumps and clusters together owing to gravity, but the individual bound structures speed away from one another relentlessly on the largest scales. As time goes on, disturbingly, each clump will disappear from view of all the others.

It's been nearly a century since scientists first theorized that the Universe was expanding, and that the farther away a galaxy was from us, the faster it appears to recede. This isn't because galaxies are physically moving away from us, but rather because the Universe is full of gravitationally-bound objects, and the fabric of space that those objects reside in is expanding.

But this picture, which held sway from the 1920s onward, has been recently revised. It's been only 20 years since we first realized that this expansion was speeding up, and that as time goes on, individual galaxies will appear to recede away from us faster and faster. In time, they'll become unreachable, even if we journeyed towards them at the speed of light. The Universe is disappearing, and there's nothing we can do about it.

The Milky Way, as seen at La Silla observatory, is a stunning, awe-inspiring sight to anyone, and . [+] offers a spectacular view of a great many stars in our galaxy. Beyond our galaxy, however, are trillions of others, nearly all of which are expanding away from us.

When you look out at a star whose light arrives after traveling towards you for 100 years, you're seeing a star that's 100 light years away, due to the fact that the speed of light is finite. But when you look out at a galaxy whose light arrives after traveling towards you for a journey of 100 million years, you're not looking at a galaxy that's 100 million light years distant. Rather, you're seeing a galaxy that's significantly farther away than that!

The reason for this is that on the largest scales — ones that aren't gravitationally bound together into galaxies, groups or clusters — the Universe is expanding. The longer it takes a photon to travel from a distant galaxy to your eyes, the greater the role of the Universe's expansion, implying that the most distant galaxies are even farther away than the amount of time the light from them has been traveling.

The farther a galaxy is, the faster it expands away from us, and the more its light appears . [+] redshifted. A galaxy moving with the expanding Universe will be even a greater number of light years away, today, than the number of years (multiplied by the speed of light) that it took the light emitted from it to reach us.

Larry McNish of RASC Calgary Center

This shows up as a cosmic redshift. Since light is emitted with a particular energy, and hence a particular wavelength, we fully expect that it will arrive at its destination with a particular wavelength as well. If the fabric of the Universe were neither expanding nor contracting, but rather were constant, that wavelength would be the same. But if the Universe is expanding, the fabric of that space is stretching as shown in the video above, and hence the wavelength of that light becomes longer. The great redshifts we've observed for the most distant galaxies have absolutely verified this picture.

Distant galaxies, like those found in the Hercules galaxy cluster, are not only redshifted and . [+] receding away from us, but their apparent recession speed is accelerating. Eventually, we will cease to receive light from beyond a certain point from them.

ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute

But we can do much more than determine that the Universe has expanded and continues to expand. We can use all the information we gather to conclude how the Universe has expanded over its history, which in turn tells us what the Universe is composed of.

Once the light leaves a distant, cosmic source, the expanding Universe stretches the wavelength of that light. This leads to a redshift, where the more distant objects will have their light redshift for longer amounts of time, when different components of the Universe (like dark energy, matter, or radiation/neutrinos) were more important.

Two of the most successful methods for measuring great cosmic distances are based on either their . [+] apparent brightness (L) or their apparent angular size (R), both of which are directly observable. If we can understand the intrinsic physical properties of these objects, we can use them as either standard candles (L) or standard rulers (R) to determine how the Universe has expanded, and therefore what it's made of, over its cosmic history.

By measuring sources at a whole slew of distances, discovering their redshift and then either measuring their intrinsic vs. apparent size or their intrinsic vs. apparent brightness, we can reconstruct the entire expansion history of the Universe.

In addition, since the way the Universe expands is determined by the various types of matter and energy present within it, we can learn what our Universe is made out of:

• 68% dark energy, equivalent to a cosmological constant,
• 27% dark matter,
• 4.9% normal (protons, neutrons and electrons) matter,
• 0.1% neutrinos and antineutrinos,
• about 0.008% photons, and
• absolutely nothing else, including no curvature, no cosmic strings, no domain walls, no cosmic textures, etc.

The relative importance of different energy components in the Universe at various times in the past. . [+] Note that when dark energy reaches a number near 100% in the future, the energy density of the Universe (and, therefore, the expansion rate) will remain constant arbitrarily far ahead in time. Owing to dark energy, distant galaxies are already speeding up in their apparent recession speed from us.

Once we know what the Universe is made out of to this degree of precision, we can simply apply this to the laws of gravity (given by Einstein's General Relativity), and determine what the future fate of our Universe is. What we discovered, when we first applied this to the discovery of a dark energy-dominated Universe, was shocking.

First off, it meant that all the galaxies that weren't already gravitationally bound to us would eventually disappear from view. They would speed away from us at an ever-increasing rate as the Universe continued to expand and expand and expand, unchecked by gravitation or any other force. As time went on, a galaxy would get more distant, meaning that there was an increasing amount of space between that galaxy and ourselves. Since space continues to expand, the galaxy appears to move away at greater and greater speeds, due to the expansion of space.

The GOODS-North survey, shown here, contains some of the most distant galaxies ever observed, some . [+] of which have had their distances independently confirmed. A great many the galaxies imaged in this image are already unreachable by us, even if we left today at the speed of light.

NASA, ESA, and Z. Levay (STScI)

But there's an inevitable conclusion that this leads to that's even more disturbing. It means that, at a particular, key distance from us, the expansion of the fabric of space itself makes it so that a photon either leaving our galaxy towards a distant one or leaving a distant galaxy headed towards ours will never reach us. The expansion rate of the Universe is so great that distant galaxies become unreachable to our own, even if we were to move at the speed of light!

At present, that distance is "only" about 15 billion light years away. If you consider that our observable Universe is some 46 billion light years in radius, and that all regions of space contain (on average and on the largest scales) the same number of galaxies as one another, it means that only about 3% of the total number of galaxies in our Universe are presently reachable by us, even if we left today, and traveled at the speed of light.

The observable (yellow) and reachable (magenta) portions of the Universe, which are what they are . [+] thanks to the expansion of space and the energy components of the Universe. 97% of the galaxies within our observable Universe are contained outside of the magenta circle they are unreachable by us today, even in principle.

E. Siegel, based on work by Wikimedia Commons users Azcolvin 429 and Frédéric MICHEL

It also means that, on average, twenty thousand stars transition every second from being reachable to being unreachable. The light they emitted a second ago will someday reach us, but the light they emit this very second never will. It's a disturbing, sobering thought, but there's also a more optimistic way to view it: this is the Universe reminding us how precious every second is. It's the Universe telling us that if we ever want to travel beyond our own local group — beyond the gravitationally bound set of objects made up of Andromeda, the Milky Way and about 60 small, satellite galaxies — that every moment we delay is another opportunity being lost.

The different possible fates of the Universe, with our actual, accelerating fate shown at the right. . [+] The continued acceleration ensures that every galaxy not gravitationally bound to our own will eventually speed away from us and become not only unreachable, but invisible to us.

Of the estimated two trillion galaxies in our Universe today, only about 3% of them are still reachable from the point of view of the Milky Way. This also means that 97% of the galaxies in our observable Universe are already out of humanity's reach, owing to the accelerated expansion of the Universe caused by dark energy. Every galaxy beyond our local group, as time goes on, is destined for that same fate.

Unless we develop the capacity for intergalactic travel and head out to other galaxy groups and clusters, humanity will forever be stuck in our local group. As time goes on, our ability to even send or receive signals to what lies beyond in the great cosmic ocean will fade from view. The accelerated expansion of the Universe is relentless, and the gravity we have isn't strong enough to overcome it. The Universe is disappearing, and there's nothing we can do to stop it.

## Decaying Dark Matter

Many people have attempted to explain this discrepancy in various ways. Another explanation was proposed recently by scientists from Fermi National Accelerator Laboratory and the University of Chicago. They suggest that in the early Universe, another type of dark matter existed. This dark matter has since decayed, changing the mass density of the Universe. With less mass, there is less gravity, and the Universe would expand faster, changing the Hubble Constant.

Not just any type of dark matter would do. Remember Cold Dark Matter? It’s cold because it moves slowly. But adding a new type of cold dark matter into the early Universe wouldn’t work - it would throw off the development of structures and galaxies within the Universe. But adding a warm dark matter - one that moves slightly faster - might work.

This warm dark matter could be made of sterile neutrinos. Eventually, this dark matter could decay into something even stranger. This would change the mass density of the Universe, leading to the discrepancy seen when measuring the Hubble Constant.

The introduction of decaying warm dark matter doesn’t completely solve the problem with the Hubble Constant, but it makes it less of a problem. If decaying warm dark matter does exist, it would change our model and our cosmology. It would illustrate that our Universe not only has strange components now, but had strange components in the past - components that may no longer exist. And to figure out that they did exist? Well, we will have to do some sleuthing. The Universe is not so quick to give up its secrets.

## Are objects in the universe moving away from each other at the same acceleration? - Astronomy

I've been out of college and grad school for a number of years now, but one little question that popped into my head during freshman astronomy still vexes me. If we can use red shift to determine the rate and direction at which objects are moving away from Earth, couldn't we take a sampling of objects and extrapolate from their speed and movement the origin, or point in space, from which they are travelling, ie, the origination point of the Big Bang? I read the other question that explains how all objects are moving away from each other and how space is expanding, but that doesn't account for the fact that the big bang is always described as this tiny point of super condensed matter. I'm thinking that extrapolating redshifts could point us back to that pinpoint of matter. Thanks for your time.

The Big Bang is often described as a tiny bit of matter, but that's an oversimplification. If the Big Bang occurred in a specific point in space, spewing galaxies in all directions, then we would expect our galaxy to be one of many galaxies sitting on an expanding shell of galaxies, with the center of that shell being the point of the "Bang." This, however, is not what we see, and not what the BB predicts.

If we were on a shell of galaxies, we would see many galaxies when we looked in directions along the shell, and few galaxies when we looked perpendicular to (up out of or down into) the shell. Moreover, distances and redshifts in such a scenario would depend on the direction we were looking. As we looked tangent to the shell, we would see many nearby galaxies with small redshifts. As we looked down into the shell, we would see more distant galaxies with higher redshifts. (Up out of the shell we would see only empty space.) This is not what we see. Galaxies, distant and nearby, are evenly distributed all around us. The number of galaxies and their redshifts are completely independant of which direction we look (we say that they are "homogeneous"), and that homogeneous distribution is also "isotropic," meaning that no matter where in the univerese you were, you would see exactly the same average distribution of galaxies and redshifts.

No, that little point of matter that was the Big Bang was not a little point of stuff inside an empty universe. It was, in fact, the entire observable universe. There was no "outside" of that point into which it could explode. In fact, the Big bang was not an explosion at all it was simply the very hot state of the early universe. Distances between objects were much shorter back then, but the universe was still homogeneous and isotropic. Wherever you were in the early universe, you would see a homogeneous, even, distribution of matter and energy around you. There was no empty "space" outside of this point of matter into which it could expand, for all of space was already there, in that little "point." The expansion of the universe is manifested only in the stretching of space itself, perpetually increasing distances between distant objects, not in some "empty space" gradually getting filled as matter streams into it. These distances expand in all directions equally, and so cannot be traced back to a single point. If you try to do this, you find that the single point is your telescope, no matter where in the universe you observe from. After all, the "point" in question was all there was of space: the entire observable universe. The Big Bang happened everywhere. It happened right where you are sitting, where the Andreomeda galaxy is now, and in the most distant reaches of the universe. It's just that the reaches of the universe were not quite as distant those many billions of years ago.

This page was last updated June 27, 2015.

### About the Author

#### Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

## What Is The Big Rip?

Dr. Thad Szabo is a professor of physics and astronomy at Cerritos College. He’s also a regular contributor to many of our projects, like the Virtual Star Party and the Weekly Space Hangout. Thad has an encyclopedic knowledge of all things space, so we got him to explain a few fascinating concepts.

In this video, Thad explains the strange mystery of dark energy, and the even stranger idea of the Big Rip.

What is the ‘Big Rip?’

If we look at the expansion of the universe, at first it was thought that, as things are expanding while objects have mass, the mass is going to be attracted to other mass, and that should slow the expansion. Then, in the late 1990’s, you have the supernova surveys that are looking deeper into space than we’ve ever looked before, and measuring distances accurately to greater distances than we’ve ever seen before. Something really surprising came out, and that was what we’ll now use “dark energy” now to explain, and that is that the acceleration is not actually slowing down – it’s not even stopped. It’s actually getting faster, and if you look at the most distant objects, they’re actually moving away from us and the acceleration is increasing the acceleration of expansion. This is actually a huge result.

One of the ideas of trying to explain it is to use the “cosmological constant,” which is something that Einstein actually introduced to his field equations to try to keep the universe the same size. He didn’t like the idea of a universe changing, so he just kind of cooked up this term and threw it into the equations to say, alright, well if it isn’t supposed to expand or contract, if I make this little mathematical adjustment, it stays the same size.

Hubble comes along about ten years later, and is observing galaxies and measuring their red shifts and their distances, and says wait a minute – no the universe is expanding. And actually we should really credit that to Georges Lemaître, who was able to interpret Hubble’s data to come up with the idea of what we now call the Big Bang.

So, the expansion’s happening – wait, it’s getting faster. And now the attempt is to try to understand how dark energy works. Right now, most of the evidence points to this idea that the expansion will continue in the space between galaxies. That the forces of gravity, and especially magnetism and the strong nuclear force that holds protons and neutrons together in the center of an atom, would be strong enough that dark energy is never going to be able to pull those objects apart.

However, there’s a possibility that it doesn’t work like that. There’s actually a little bit of experimental evidence right now that, although it’s not well-established, that there’s a little bit of a bias with certain experiments that dark energy may get stronger over time. And, if it does so, the distances won’t matter – that any object will be pulled apart. So first, you will see all galaxies recede from each other, as space starts to grow bigger and bigger, faster and faster. Then the galaxies will start to be pulled apart. Then star systems, then planets from their stars, then stars themselves, and then other objects that would typically be held together by the much stronger forces, the electromagnetic force objects held by that will be pulled apart, and then eventually, nuclei in atoms.

So if dark energy behaves so that it gets stronger and stronger over time, it will eventually overcome everything, and you’ll have a universe with nothing left. That’s the ‘Big Rip’ – if dark energy gets stronger and stronger over time, it will eventually overcome any forces of attraction, and then everything is torn apart.

## Is the universe actually shrinking?

Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.

Whoa, here's something to think about. Maybe the Universe isn't expanding at all. Maybe everything is actually just shrinking, so it looks like it's expanding. Turns out, scientists have thought of this.

There are some people who would have you believe the Universe is expanding. They're peddling this idea it all started with a bang, and that expansion is continuing and accelerating. Yet, they can't tell us what force is causing this acceleration. Just "dark energy", or some other JK Rowling-esque sounding thing. Otherwise known as the acceleration that shall not be named, and it shall be taught in the class which follows potions in 3rd period.

I propose to you, faithful viewer, an alternative to this expansionist conspiracy. What if distances are staying the same, and everything is in fact, shrinking? Are we destined to compress all the way down to the Microverse? Is it only a matter of time before our galaxy starts drinking its coffee from a thimble or perhaps sealed in a pendant hanging on Orion's belt? So, could we tell if that's actually what's going on?

The first horrible and critical assumption here is that shrinking objects and an expanding universe would look exactly the same, which without magic or handwaving just isn't the case. But you don't have to take my word for it, we have science to punch holes in our Shrink-truther conspiracy.

Let's start with distances. If we assumed the Earth and everything on it was getting smaller, we'd also be shrinking things like meter sticks. In the past they would have been larger. If everything was larger in the past, including the length of a meter, this means the speed of light would have appeared slower in the past. So was the speed of light slower in the past? I'm afraid it wasn't, which really hobbles the shrinky-dink universe plot. But how do we know that?

You've probably seen spectral lines before or at least heard them referenced. Scientists use them to determine the chemical composition of materials. A changing speed of light would affect the spectral lines of distant objects, and because some people are just super smart and were able to do the math on this, we know that when we look at distant gas clouds we find the speed of light has changed no more than one part in a billion over the past 7 billion years.

Shrinking objects would also become more dense over time. This means that the universal constant of gravity should appear smaller in the past. Some have actually studied this, to determine whether it has changed over time, and they've also seen no change.

The diagram shows the electromagnetic spectrum, the absorption of light by the Earth’s atmosphere and illustrates the astronomical assets that focus on specific wavelengths of light. ALMA at the Chilean site and with modern solid state electronics is able to overcome the limitations placed by the Earth’s atmosphere. Credit: Wikimedia, T.Reyes

If objects in the Universe were shrinking, the Universe would actually be collapsing. If galaxies weren't moving away from each other, their gravity would cause them to start falling toward each other. If they were shrinking, assuming their mass doesn't change, their gravity would be just as strong, so shrinking wouldn't stop their mutual attraction. A Universe of shrinking objects would look exactly opposite to what we observe.

So, good news. We're pretty sure that objects, and us, and all other things in the Universe are not shrinking. We're still not sure why anyone would name a thing Shrinky Dinks. Especially a craft toy marketed at children.

Artists illustration of the expansion of the Universe. Credit: NASA, Goddard Space Flight Center

## Dark energy explained by relativistic time dilation?

Timeline of the universe. Image credit: NASA/WMAP Science Team In the recent Hollywood film “Interstellar,” a team of scientists travel through a wormhole in space to access planets with promising conditions to sustain life on Earth. One of the issues the team must grapple with is time dilation: each hour spent collecting data on a given planet is equal to seven years on Earth.

Einstein’s general theory of relativity indicates that time dilation in response to gravity is directional in that an object in high gravity will have slower time than an object in low gravity. In contrast, Einstein’s theory of special relativity describes reciprocal time dilation between two moving objects, such that both moving objects’ times appear to be slowed down relative to each other.

This new paper makes the case that instead of being reciprocal, time dilation in response to movement is directional, with only the moving object undergoing time dilation.

The study, “Implication of an Absolute Simultaneity Theory for Cosmology and Universe Acceleration,” was published 23d December 2014 in the journal PLOS ONE.

A molecular geneticist whose lab works on cell cycle regulation, Professor Edward Kipreos became interested in cosmology and the theory of special relativity several years ago. Image credit: University of Georgia A molecular geneticist whose lab works on cell cycle regulation, Professor Edward Kipreos became interested in cosmology and the theory of special relativity several years ago. He says the phenomenon can be easily understood in the context of how Global Positioning System satellites work.

“The satellites, which travel in free-fall reference frames, are moving fast enough, in relation to the Earth, that you have to correct for their time being slowed down, based on their speed,” said Kipreos. “If we didn’t correct for that, then the satellites’ GPS measurement would be off by a factor of two kilometres per day.”

This simple example &mdash GPS satellites sending out the time, which is then detected back on Earth, where the distance between the two is measured &mdash is based on the theory of special relativity and the Lorentz Transformation, a mathematical map that describes how measurements of space and time by two observers are related.

“Special relativity is supposed to be reciprocal, where both parties will experience the same time dilation, but all the examples that we have right now can be interpreted as directional time dilation,” Kipreos said. “If you look at the GPS satellites, the satellite time is slowing down, but according to the GPS satellites, our time is not slowing down &mdash which would occur if it were reciprocal. Instead, our time is going faster relative to the satellites, and we know that because of constant communication with the satellites.”

An alternative theory, the Absolute Lorentz Transformation, describes directional time dilation. Kipreos found that this theory is compatible with available evidence if the “preferred reference frame” for the theory, relative to which directional time dilation occurs, is linked to centres of gravitational mass. Near the Earth, the preferred reference frame would be the “Earth-centered non-rotating inertial reference frame,” which is currently used to calculate the time dilation of GPS satellites.

“A strict application of the Absolute Lorentz Transformation to cosmological data has significant implications for the universe and the existence of dark energy,” Kipreos said.

As the universe gets larger, cosmological objects, such as galaxies, move more rapidly away from each other in a process known as Hubble expansion. The Absolute Lorentz Transformation indicates that increased velocities induce directional time dilation. Applying this to the increased velocities associated with Hubble expansion in the present universe suggests a scenario in which the present experiences time dilation relative to the past. The passage of time would therefore be slower in the present and faster in the past.

Supernovae that explode with the same intensity are used as “standard candles” to measure cosmological distances based on how bright they appear. Those that are relatively close to the Earth line up on a plot of distance (based on the redshift of light) and brightness. However, in 1998 and 1999, the observation that supernovae at greater distances are fainter than would be expected provided evidence that the rate of universe expansion has accelerated recently.

“The accelerated expansion of the universe has been attributed to the effects of dark energy,” Kipreos said. “However, there is no understanding of what dark energy is or why it has manifested only recently.

“The predicted effects of time being faster in the past would have the effect of making the plot of supernovae become linear at all distances, which would imply that there is no acceleration in the expansion of the universe. In this scenario there would be no necessity to invoke the existence of dark energy.”

## Speed of Universe's Expansion Measured Better Than Ever

The most precise measurement ever made of the speed of the universe's expansion is in, thanks to NASA's Spitzer Space Telescope, and it's a doozy. Space itself is pulling apart at the seams, expanding at a rate of 74.3 plus or minus 2.1 kilometers (46.2 plus or minus 1.3 miles) per second per megaparsec (a megaparsec is roughly 3 million light-years).

If those numbers are a little too much to contemplate, rest assured that's really, really fast. And it's getting faster all the time.

American astronomer Edwin P. Hubble first discovered that our universe isn't static in the 1920s. In fact, Hubble found, space has been expanding since it began with the Big Bang 13.7 billion years ago. Then, in the 1990s, astronomers shocked the world again with the revelation that this expansion is speeding up (this discovery won its finders the 2011 Nobel Prize in physics).

Ever since Hubble's initial discovery, scientists have been trying to refine their measurement of the universe's expansion rate, called the Hubble Constant. It's a hard measurement to make.

The new value reduces the uncertainty in the Hubble Constant to just 3 percent, and improves the precision of the measurement by a factor of 3 compared to a previous estimate from the Hubble Space Telescope.

"Just over a decade ago, using the words 'precision' and 'cosmology' in the same sentence was not possible, and the size and age of the universe was not known to better than a factor of two," Wendy Freedman of the Observatories of the Carnegie Institution for Science in Pasadena, Calif., said in a statement. "Now we are talking about accuracies of a few percent. It is quite extraordinary." [7 Surprising Facts About the Universe]

The new measurement doesn't just tell scientists how fast the universe is expanding, but helps shed light on the mystery of why this expansion is accelerating. Dark energy is the name given to whatever is causing the universe's expansion to speed up. Yet scientists have little idea what it is.

By combining the new value of the Hubble Constant with observations of the universe by NASA's Wilkinson Microwave Anisotropy Probe (WMAP), the scientists were able to make an independent calculation of the strength of dark energy, which is battling against gravity to pull the universe outward.

"This is a huge puzzle," Freedman said. "It's exciting that we were able to use Spitzer to tackle fundamental problems in cosmology: the precise rate at which the universe is expanding at the current time, as well as measuring the amount of dark energy in the universe from another angle."

Spitzer spies the universe in long-wavelength infrared light not visible to the human eye, which allowed it to peer through obscuring dust to the distant universe. The telescope focused on variable stars called cepheids, which are reliable distance indicators because their intrinsic brightness can be calculated based on their pulsing light. If their intrinsic brightness is known, then their distance can be estimated by comparing their apparent brightness, because the farther away stars are, the more their light dims.

"These pulsating stars are vital rungs in what astronomers call the cosmic distance ladder: a set of objects with known distances that, when combined with the speeds at which the objects are moving away from us, reveal the expansion rate of the universe," said Glenn Wahlgren, Spitzer program scientist at NASA Headquarters in Washington.

Spitzer observed 90 cepheid stars, and was able to measure their apparent brightness more precisely than previous studies, leading the way to a more refined measurement of their distances, and the expansion rate of space.

The Spitzer telescope was launched in August 2003, and ran out of cryogenic coolant to chill its instruments in May 2009. Without coolant, the observatory can't see in all the wavelengths it was originally designed for. Since then, however, Spitzer's been running on a second, "warm" mission that's proven fruitful as well.

The new findings are reported in a paper published in the Astrophysical Journal.

## Is Everything in the Universe Expanding?

The Universe is expanding. Distant galaxies are moving away from us in all directions. It’s natural to wonder, is everything expanding? Is the Milky Way expanding? What about the Solar System, or even objects here on Earth. Are atoms expanding?

Nope. The only thing expanding is space itself. Imagine the Universe as loaf of raisin bread rising in the oven. As the bread bakes, it’s stretching in all directions – that’s space. But the raisins aren’t growing, they’re just getting carried away from each other as there’s more bread expanding between them.

Space is expanding from the Big Bang and the acceleration of dark energy. But the objects embedded in space, like planets, stars, and galaxies stay exactly the same size. As space expands, it carries galaxies away from each other. From our perspective, we see galaxies moving away in every direction. The further galaxies are, the faster they’re moving.

There are a few exceptions. The Andromeda Galaxy is actually moving towards the Milky Way, and will collide with us in about 4 billion years.In this case, the pull of gravity between the Milky Way and Andromeda is so strong that it overcomes the expansion of the Universe on a local level.

Within the Milky Way, gravity holds the stars together, and same with the Solar System. The nuclear force holding atoms together is stronger than this expansion at a local scale. Is this the way it will always be? Maybe. Maybe not.

A few decades ago, astronomers thought that the Universe was expanding because of momentum left over from the Big Bang. But with the discovery of dark energy in 1998, astronomers realized there was a new possibility for the future of the Universe. Perhaps this accelerating dark energy might be increasing over time.

In billions years from now, the expansive force might overcome the gravity that holds galaxies together. Eventually it would become so strong that star systems, planets and eventually matter itself could get torn apart.This is a future for the Universe known as the Big Rip. And if it’s true, then the space between stars, planets and even atoms will expand in the far future.

Is this going to happen? Astronomers don’t know. Their best observations so far can’t rule it out, or confirm it. And so, future observations and space missions will try to calculate the rate of dark energy’s expansion.

So no, matter on a local level isn’t expanding. The spaces between planets and stars isn’t growing. Only the distances between galaxies which aren’t gravitationally bound to each other is increasing. Because space itself is expanding.

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