Astronomy

What's the density of dark matter (clouds)?

What's the density of dark matter (clouds)?

How much mass per unit volume do known dark matter have, for instance in the Bullet Cluster? Does the density vary in space, for example, do individual dark matter structures have denser cores?


Dark clouds in space

An infrared image of an infrared dark cloud, seen against a glowing background of warm infrared dust. This IRDC has a young star forming within (seen as green spot). Credit: NASA Spitzer

(PhysOrg.com) -- Infrared dark clouds (IRDCs) are dark patches in the sky seen against the continuous, bright infrared background produced by our galaxy. IRDCs are rich in molecules and relatively dense, cool gas, and they are natural sites for future star birth. Studies of IRDCs to date have emphasized those candidates that already have star formation underway within them, but astronomers are increasingly interested in probing younger, colder clouds to probe earlier stages in the star formation process.

One tool to use is the gas, ammonia. In 1969, astronomers discovered that ammonia (NH3) was present in large quantities in interstellar gas clouds. The species was most apparent in regions of star formation where the density and temperature of the gas enabled it to emit bright radio-wavelength radiation. Since then, ammonia has become one of the staple diagnostic probes of the regions where new stars are forming. One issue has been that radio telescopes capable of detecting ammonia have relatively poor spatial discrimination this means many IRDCs appear as point sources, without structure.

CfA astronomer David Wilner and two colleagues teamed up to use a combined set of telescopes: one group in New Mexico and the second one in West Virginia. Operating together coherently, the combination is able to see small sub-structures in IRDCs, including regions within the clouds where new stars may be forming.

The group reports on a set of six relatively young IRDCs in the new issue of the Astrophysical Journal.

They find strong signals from ammonia, and calculate from them that the gas temperatures are only about ten degrees above absolute zero. Although a few of their sample of IRDCs are known to contain young stars, the ammonia gas in all cases was cold - apparently the new stars have not heated up the cloud. Of particular importance is their conclusion that the density of the gas is high (none has frozen out onto dust, for example, as can happen to molecules like carbon monoxide). The paper concludes from the overall physical conditions that these clouds are probably resistant to collapsing into new stars. New stars do form in them, they argue, largely because of pressure from the much warmer gas reservoir surrounding them in their galactic neighborhoods.


Pulsar measurements reveal new information about the density of dark matter

Researchers have used pulsar experiments to help learn new knowledge about the abundance of dark matter in our home galaxy. In a new study, led by the Rochester Institute of Technology Associate Professor Sukanya Chakrabarti, to be published in the Astrophysical Journal Letters, researchers have now obtained the first direct measurement of the average acceleration taking place in the Milky Way.

It is well known that the expansion of the universe is accelerating as a result of unexplained dark energies. Stars also undergo movement inside galaxies, and this is due to a mix of dark matter and stellar mass. In a recent report to be published in the Astrophysical Journal Letters, researchers have now obtained the first precise calculation of the average acceleration taking place in our home galaxy, the Milky Way. These calculations will guide the direct detection of dark matter experiments carried out in laboratories around the world.

First direct measurement of the average acceleration taking place within our home galaxy. Based on these new high-precision measurements and the known amount of visible matter in the galaxy, researchers were then able to calculate the Milky Way’s dark matter density without making the usual assumption that the galaxy is in a steady-state.

Led by Sukanya Chakrabarti at the Center for Advanced Research with collaborators from the Rochester Institute of Technology, the University of Rochester, and the University of Wisconsin-Milwaukee, the team used pulsar data to clock the radial and vertical acceleration of the stars within and outside the galactic plane. Based on these recent high-precision measurements and the known volume of visible matter in the galaxy, researchers were then able to determine the dark matter mass of the Milky Way without the normal presumption that the galaxy is in a steady state.

Milky Way’s Dark Side Revealed by Measurements of Pulsar Acceleration

“Our analysis not only gives us the first measurement of the tiny accelerations experienced by stars in the galaxy, but also opens up the possibility of extending this work to understand the nature of dark matter, and ultimately dark energy on larger scales,” stated Chakrabarti, the paper’s lead author and a current Member and IBM Einstein Fellow at the Institute for Advanced Study.

Stars are hovering around the world at hundreds of kilometers per second, and this analysis shows that their velocity is changing at a literal slow pace—a few centimeters per second, which is around the same speed as a crawling infant. To detect this slight motion, the research team focused on the ultra-precise time-keeping capabilities of pulsars that are widely scattered around the galactic plane and halo—a dispersed spherical field that surrounds the galaxy.

“By exploiting the unique properties of pulsars, we were able to measure very small accelerations in the Galaxy. Our work opens a new window in galactic dynamics,” said co-author Philip Chang of the University of Wisconsin-Milwaukee.

Extending about 300,000 light-years from the galactic center, the halo may offer valuable insights into dark matter, which accounts for around 90 percent of the galaxy’s mass and is heavily concentrated above and below the star-dense galactic plane. Stellar motion in this specific region—the primary subject of this study—may be affected by dark matter. Using the local density measurements obtained from this analysis, researchers will now have a clearer understanding of how and when to look for dark matter.

Although previous studies have assumed a state of galactic harmony in order to measure average mass density, this study is focused on the normal, non-equilibrium state of the galaxy. This may be analogized to the contrast between the surface of the pond before and after the stone is thrown in. The team was able to obtain a more realistic view of fact by allowing for the “ripples” But in this situation, rather than stones, the Milky Way is affected by a tumultuous background of interstellar mergers and tends to be disrupted by external dwarf galaxies such as the Small and Wide Magellanic Clouds.

As a result, the stars do not have smooth orbits and appear to follow a course close to that of a distorted vinyl album, traveling above and below the galactic plane. One of the key factors that made this direct observational approach possible was the use of pulsar data compiled from international collaborations, including NANOGrav (North American Nanohertz Observatory for Gravitational Wave) which collected data from the Green Bank and Arecibo telescopes.

“For centuries astronomers have measured the positions and speeds of stars, but these provide only a snapshot of the complex dynamical behavior of the Milky Way galaxy,” stated Scott Tremaine, Professor Emeritus at the Institute for Advanced Study. “The accelerations measured by Chakrabarti and her collaborators are directly caused by the gravitational forces from the matter in the galaxy, both visible and dark, and thereby provide a new and promising window on the distribution and the composition of the matter in the galaxy and the universe.”

This exclusive paper would allow for a wide range of potential research. Accurate acceleration measurements will also soon be possible using the complementary radial velocity method developed earlier this year by Chakrabarti, which calculates the difference in the velocity of stars with high precision. This work would also allow more comprehensive simulations of the Milky Way, improve restrictions on general relativity, and provide hints in the quest for dark matter. In the end, extensions of this approach can allow us to calculate cosmic acceleration directly as well.

Though a clear image of our home galaxy-similar to those of the Planet captured by the Apollo astronauts-is not yet feasible, this analysis offered important new information to help imagine the complex organization of the galaxy from within. This work would also allow more comprehensive simulations of the Milky Way, improve restrictions on general relativity, and provide hints in the quest for dark matter. In the end, extensions of this approach can allow us to calculate cosmic acceleration directly as well.


Where Is Dark Matter Most Dense? Subaru Telescope Gets Some Hints

Put another checkmark beside the “cold dark matter” theory. New observations by Japan’s Subaru Telescope are helping astronomers get a grip on the density of dark matter, this mysterious substance that pervades the universe.

We can’t see dark matter, which makes up an estimated 85 percent of the universe, but scientists can certainly measure its gravitational effects on galaxies, stars and other celestial residents. Particle physicists also are on the hunt for a “dark matter” particle — with some interesting results released a few weeks ago.

The latest experiment with Subaru measured 50 clusters of galaxies and found that the density of dark matter is largest in the center of these clusters, and smallest on the outskirts. These measurements are a close match to what is predicted by cold dark matter theory, scientists said.

Cold dark matter assumes that this material can’t be observed in any part of the electromagnetic spectrum, the band of light waves that ranges from high-energy X-rays to low-energy infrared heat. Also, the theory dictates that dark matter is made up of slow-moving particles that, because they collide with each other infrequently, are cold. So, the only way dark matter interacts with other particles is by gravity, scientists have said.

To check this out, Subaru peered at “gravitational lensing” in the sky — areas where the light of background objects are bent around dense, massive objects in front. Galaxy clusters are a prime example of these super-dense areas.

Several dark matter maps: one based on a sample of 50 individual galaxy clusters (left), another looking at an average galaxy cluster (center), and another based on dark matter theory (right). Red is the highest concentration of dark matter, followed by yellow, green and blue. At right, in the middle, is a map based on cold dark matter theory that comes close to the average galaxy cluster observed with the Suburu Telescope. Credit: NAOJ/ASIAA/School of Physics and Astronomy, University of Birmingham/Kavli IPMU/Astronomical Institute, Tohoku University)

“The Subaru Telescope is a fantastic instrument for gravitational lensing measurements. It allows us to measure very precisely how the dark matter in galaxy clusters distorts light from distant galaxies and gauge tiny changes in the appearance of a huge number of faint galaxies,” stated Nobuhiro Okabe, an astronomer at Academia Sinica in Taiwan who led the study.

Next, the team members could compare where the matter was most dense with that predicted by cold dark matter theory. To do that, they measured 50 of the most massive, known clusters of galaxies. Then, they measured the “concentration parameter”, or the cluster’s average density.

“They found that the density of dark matter increases from the edges to the center of the cluster, and that the concentration parameter of galaxy clusters in the near universe aligns with CDM theory,” stated the National Astronomical Observatory of Japan.

The next step, researchers stated, is to measure dark matter density in the center of the galaxy clusters. This could reveal more about how this substance behaves. Check out more about this study in Astrophysical Journal Letters.


The light side of dark matter

Although dark matter cannot be seen, it can be studied by the gravitational effect of dark objects on the light from background stars. New observations of the nearby Andromeda galaxy probe the possibility that the dark matter could be small black holes.

Dark matter is usually assumed to be some form of elementary particle, but this is as yet undetected. Attention has therefore turned to another dark matter candidate: black holes. Neither stellar black holes nor the supermassive ones in galactic nuclei could make up the entirety of dark matter, because they derive from ordinary baryons, and the cosmological nucleosynthesis scenario implies that these can account for at most 20% of the dark matter density. However, it is possible that primordial black holes (PBHs) were generated during the radiation-dominated era of the early Universe, well before cosmological nucleosynthesis, and these would be non-baryonic. Hiroko Niikura and collaborators, in a recent paper published in Nature Astronomy 1 , constrain this scenario by looking for such PBHs in the halo of the Milky Way or the Andromeda galaxy.


What is the mechanism by which molecular clouds collapse to form stars?

Obviously the answer is gravity, but I'm having a hard time understanding the process by which the collapse starts.

I looked it up and molecular clouds have a molecules/cm^3 density of between 10^2 and 10^6. While far denser than the interstellar medium overall, that's still very diffuse.

Where I'm struggling is being able to visualize how a hundred or even a million H2 molecules in a cubic centimeter are exerting enough of gravitational effect to overcome the kinetic energy of gas molecule collisions and initiate a collapse.

I've read the wiki article and other articles I could find, and there's mention of how the extremely low temperatures and presence of dust particles leads to clumping. However, it still seems to me that barring some external factor, those molecular clouds should become more diffuse through space rather than collapsing down.

#2 Araguaia

Where I'm struggling is being able to visualize how a hundred or even a million H2 molecules in a cubic centimeter are exerting enough of gravitational effect to overcome the kinetic energy of gas molecule collisions and initiate a collapse.

They can't, but a zillion H2 molecules in a few cubic light-years can. Their collective mass, though spread out, forms a sort of broad shallow depression in space-time, towards the middle of which the molecules very slowly bu inexorably flow.

As the density increases near the center the dip in space-time becomes deeper, until stuff is downright falling in, and starting to spin faster as it does.

#3 mvas

Where I'm struggling is being able to visualize how a hundred or even a million H2 molecules in a cubic centimeter are exerting enough of gravitational effect to overcome the kinetic energy of gas molecule collisions and initiate a collapse.

I've read the wiki article and other articles I could find, and there's mention of how the extremely low temperatures and presence of dust particles leads to clumping. However, it still seems to me that barring some external factor, those molecular clouds should become more diffuse through space rather than collapsing down.

What am I missing?

Gravity is relentless, it never stops.

Locally, gravity will continuously bias the "random" movements from the kinetic energy, towards the center of gravity of the cloud.

Edited by mvas, 08 October 2018 - 08:07 AM.

#4 Luca Brasi

#5 David E

Obviously the answer is gravity, but I'm having a hard time understanding the process by which the collapse starts.

I looked it up and molecular clouds have a molecules/cm^3 density of between 10^2 and 10^6. While far denser than the interstellar medium overall, that's still very diffuse.

Where I'm struggling is being able to visualize how a hundred or even a million H2 molecules in a cubic centimeter are exerting enough of gravitational effect to overcome the kinetic energy of gas molecule collisions and initiate a collapse.

I've read the wiki article and other articles I could find, and there's mention of how the extremely low temperatures and presence of dust particles leads to clumping. However, it still seems to me that barring some external factor, those molecular clouds should become more diffuse through space rather than collapsing down.

What am I missing?

From what I understand about the process- there is plenty of gas to make not one but many, many stars. The slow and hard part is getting the first one started, since the fledgling star must first overcome the outward pressures of collapsing gas as it heats up from compression (and eventually ignites into fusion). But once that first one gets started the outward blast of the solar wind pushes the unused gas outward and slams it into surrounding layers of gas speeding up the star birth process for other stars. Perhaps dark matter and/or dark energy is involved somehow?


By definition, baryonic matter should only include matter composed of baryons. In other words, it should include protons, neutrons and all the objects composed of them (i.e. atomic nuclei), but exclude things such as electrons and neutrinos which are actually leptons.

In astronomy, however, the term ‘baryonic matter’ is used more loosely, since on astronomical scales, protons and neutrons are always accompanied by electrons (in appropriate numbers for astronomical objects to possess all but zero net charge). Astronomers therefore use the term ‘baryonic’ to refer to all objects made of normal atomic matter, essentially ignoring the presence of electrons which, after all, represent only

0.0005 of the mass. Neutrinos, on the other hand, are (correctly) considered non-baryonic by astronomers.

Another slight oddity in the usage of the term baryonic matter in astronomy is that black holes are included as baryonic matter. While the matter from which black holes form is mainly baryonic matter, once swallowed by the black hole, this distinction is lost. For example, a theoretical black hole constructed purely out of photons (which are bosons and clearly not baryons) is indistinguishable from one made from normal baryonic matter. This is often referred to as the ‘black holes have no hair’ theorem which simply states that black holes do not have properties such as baryonic or non-baryonic.
Objects in the Universe composed of baryonic matter include:

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ISSUES IN THE PHILOSOPHY OF COSMOLOGY

George F R Ellis , in Philosophy of Physics , 2007

2.3.6 Matter Distribution and Motion: Dark Matter

Detailed studies have been made of the distribution of galaxies and their motions. They occur in clusters, in turn making up superclusters imbedded in vast walls surrounding relatively sparsely populated intergalactic voids. The galaxy luminosity function characterizes the numbers of galaxies occurring within each luminosity class the covariance function characterizes their spatial clustering [ Peebles, 1993b Dodelson, 2003 ]. Large scale motions occur for galaxies in clusters, and for the clusters themselves. It is easy to conceive of matter that is hard to detect (for example, small rocks distributed through space) studies of galactic rotation curves and of motions of galaxies in clusters [ Bothun, 1998, pp. 139-181 ] imply the existence of huge amounts of unseen dark matter, dominating the dynamics of the Universe: its density is Ωdm0 ⋍ 0.3, much greater than both visible matter (Ωvm0 = 0.015) and baryons (Ωbar0 = 0.044), but significantly less than the critical density Ω0 = 1. Thus the dark matter is non-baryonic, meaning it has some kind of exotic nature rather than being the protons and neutrons that are the substance of ordinary matter [ Seife, 2003 ]. In contrast to the ‘dark energy’ discussed in the previous section, dark matter is dynamically effective on astrophysical scales as well as on cosmological scales. Many attempts have been made to identify its nature, for example it might be axions, supersymmetric partners of known particles, quark nuggets, or massive neutrinos [ Gribbin and Rees, 1991 Perkins, 2005 ], but what it is composed of is still unknown. Laboratory searches are under way to try to detect this matter, so far without success. A key question is whether its apparent existence is due to our using the wrong theory of gravity on these scales. This is under investigation, with various proposals for modified forms of the gravitational equations that might explain the observations without the presence of large quantities of dark matter. This is a theoretical possibility, but the current consensus is that this dark matter does indeed exist.

An important distinction is whether dark matter consists of

weakly interacting massive particles that cooled down quickly, thereafter forming cold dark matter (‘CDM’) moving slowly at the time of structure formation (and resulting in a bottom-up process with large scale structure forming from smaller scale structures), or

particles that have a low mass and cooled slowly, therefore for a long time forming hot dark matter, moving very fast at the time of structure formation (and resulting in a top-down galaxy formation scenario).

Structure formation studies currently support the CDM hypothesis, with hierarchical formation of gravitationally bound objects taking place in a complex bottom up process involving interactions of CDM, baryons, and radiation, with dwarf galaxies forming initially [ Silk, 2005 Mouri and Taniguchi, 2005 ] and then aggregating to form larger structures. These studies are based on massive numerical simulations, with initial conditions provided by the inflationary scenario discussed below, see Sec. 2.6 . Unlike ‘dark energy’, CDM has an ordinary baryonic equation of state (it is a perfect fluid (4) with pcdm = 0 ⇔ wcdm = 0).

Another way of detecting dark matter in clusters is by its gravitational lensing effects [ Schneider et al., 1992 ]. The bending of light by massive objects was one of the classic predictions of General Relativity theory. Rich clusters of galaxies or galaxy cores can cause strong lensing of more distant objects, where multiple images of distance galaxies and qso's occur, sometimes forming rings or arcs and weaker lensing by closer masses results in characteristic patterns of distortions of images of distant objects. Analysis of multiple images can be used to reconstruct the lensing mass distributions, and statistical analysis of weak lensing patterns of image distortions are now giving us detailed information on the matter distribution in distant galaxies and clusters. These studies show that to get enough lenses in an almost flat cosmology (Ω0 ≃ 1) requires the presence of a cosmological constant — there cannot be a critical density of dark matter present [ Dodelson, 2003 Silk, 2005 ].

A key feature of present-day cosmology is attempts to identify the nature of this dark matter, and if possible to detect it in a laboratory situation. While observations favour the CDM scenario, some residual problems as regards the emergence of fine-scale structure still need resolution [ Silk, 2005 ].


Less Dark Matter in Young Galaxies?

By: Monica Young March 15, 2017 2

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A new study of six young, star-forming galaxies suggests they're less influenced by dark matter than expected. But the results may say more about galaxy evolution than about the nature of dark matter.

This artist's impression compares rotating disk galaxies in the distant universe (right) and the present day (left).
ESO

If only Vera Rubin had lived another year: I wonder what she would have made of the news today. Rubin and her colleague at the Carnegie Institution of Washington, Kent Ford, achieved astronomical fame when they measured the rotation of our neighbor Andromeda Galaxy 47 years ago. Their work served as a crucial piece of evidence for the existence of dark matter.

Now, in the journal Nature, Reinhard Genzel (Max Planck Institute, Garching, Germany) and colleagues report similar measurements of six distant galaxies — with a result surprisingly opposed to Rubin’s historic find.

Discovering Dark Matter

When Rubin and Ford collected spectra of ionized hydrogen in Andromeda almost half a century ago, they measured the speed of 67 gas clouds as they whirled about the galaxy’s center with far greater precision than ever before. What the astronomers found was at the time quite curious: beyond 15,000 light-years or so from the galaxy’s center, the clouds’ velocities didn’t slow down — the outermost clouds whirled just as fast as those much closer to the center. Either Andromeda Galaxy was in the midst of flying apart (not likely) or there was some additional matter in the galaxy’s outer reaches that we just couldn’t see.

This groundbreaking result, though not the first to suggest the existence of dark matter, encouraged scientists to start taking the matter seriously. And even though physicists still struggle to detect dark matter particles in the lab, astronomers have had enormous success in supporting their existence.

Galaxy clusters also show evidence of dark matter. Distorted galaxies rim the edges of the gravitational lens Abell 1689, a galaxy cluster 2.2 billion light-years distant in Virgo. The purple overlay on this Hubble Space Telescope image shows the distribution of dark matter within the cluster as determined from the effect of weak gravitational lensing.

Since Rubin and Ford’s 1970 publication, scientists have found multiple lines of evidence for dark matter, such as the rotations of galaxies within clusters, weak gravitational lensing, and incredibly large-scale computer simulations of the distribution of galaxies in the universe. These observations suggest that galaxies and even galaxy clusters are ensconced in gigantic, massive dark matter halos, which started coming together before the stars began to shine.

That’s why the six galaxies studied by Genzel’s team proved so surprising.

Missing Halos

One of the six galaxies that Genzel and colleagues studied. The left frame shows a false-color representation of the galaxy's hydrogen. The right frame shows the shift of the hydrogen alpha line, which the team used to determine the galaxy's rotation.
MPE

Genzel and colleagues observed several hundred star-forming galaxies in the distant universe (2.5 billion to 8 billion years after the Big Bang) using the European Southern Observatory’s Very Large Telescope. The galaxies are Milky Way-mass or more, which is pretty massive considering that we’re looking billions of years back in time. The galaxies are also forming 50 to 200 Suns’ worth of stars a year, a typical rate of star formation for this cosmic era.

Like Rubin and Ford, Genzel’s team measured the motion of hydrogen gas clouds. Unlike Rubin and Ford, the new measurements showed that toward the edge of six massive, star-forming galaxies, the clouds did slow down. Averaged data from 97 other (fainter) galaxies show the same result.

That’s not to say there isn’t some dark matter there — just not as much as expected. The dark matter cushions these galaxies lounge in appear to be rather threadbare.

This artist's impression compares rotating disk galaxies in the distant universe (right) and the present day (left).
ESO

Evolution of Galaxies and Halos

It turns out these results may say more about the path of galaxy evolution than about the nature of dark matter. In fact, computer simulations of dark matter may even have predicted what Genzel and colleagues observed.

One possibility, says Mark Swinbank (Durham University, UK), who authored an opinion piece accompanying the Nature article, is that the dark matter halos of these galaxies are still in the process of growing. But that would fundamentally change how we view galaxy evolution, where the standard picture says that the halos are largely in place before the gas and stars come together.

Another possibility is that we’re simply viewing these galaxies during a crucial era. Genzel’s team chose to observe massive, star-forming disk galaxies during “cosmic noon,” the universe’s peak in star formation. These are the ancient precursors to “red and dead” elliptical galaxies we see nearer the Milky Way, so nicknamed for their redder color and their low rates of star formation. Recent computer simulations by Adi Zolotov (The Hebrew University, Israel, and Ohio State University) and colleagues, show that virtually all such massive galaxies take the fast track toward evolution, their journey instigated by a single event.

Whether it be a merger with another galaxy or gas flows entering the galaxy from the larger cosmic web, this event triggers a burst of star formation in the galaxy’s center. As a result, massive, star-forming galaxies during this cosmic era will look a lot more compact than they actually are — “blue nuggets,” as Zolotov and colleagues refer to them. So measuring nuggets’ rotation won’t reveal the full dark matter halo around them, because observations would only cover the parts of the galaxies that are dominated by normal matter.

“[Genzel and colleagues’] declining rotation curves in massive star-forming galaxies are just what the high-resolution zoom-in galaxy simulations by my collaborators and I predicted,” says Joel Primack (University of California, Santa Cruz), a coauthor on Zolotov’s paper.

A Matter of Resolution

It’s worth noting that other simulations, such as Illustris and Eagle, don’t make the same prediction, but Primack points out that this could be due their fuzzier view. Simulating an entire universe is a battle between resolution, volume, and time covered versus computing time. While the Illustris and Eagle simulations can see elements down to 3,000 light-years across (they can’t make out star formation regions, for example), the more computationally expensive simulations that Primack and Zolotov are involved in can see details as fine as 60 light-years.

“Both are useful,” Primack says, “but to find out what’s really going on inside these galaxies, you really have to simulate these high-resolution environments.”


Dark energy epoch

© 2005 Pearson Prentice Hall, Inc

This diagram illustrates the timeline for the radiation, matter and dark energy dominance in the universe. At the present time, we are in the dark energy epoch. The radiation density and matter density both continued to fall as the universe expanded, but dark energy density remained constant. Eventually, dark energy became the dominant material in the universe. This raises some interesting questions.

How can the density of dark energy remain constant?

The size of the universe keeps getting bigger and bigger. Doesn't this mean that there is more and more dark energy? Doesn't that break some kind of conservation law?

It could be that there exists a kind of potential dark energy. It would be similar in concept to gravitational potential energy. When you lift a massive object, say a bowling ball, you do work against gravity to lift it. The higher you lift it, the more work you do. That work energy is stored as potential energy. When you drop the ball, it falls to Earth, moving faster and faster. The gravitational potential energy is converted to kinetic energy of motion. The total energy is conserved it just changes form from potential to kinetic energy.

Similarly, the dark energy we see could be arising from a potential dark energy. It is not being created from nothing. It is just changing form.

Now, what about the curvature of the universe? We think we have a flat universe, but now we see that the expansion rate of the universe is increasing. Isn't that a contradiction?

How can we say that we have a flat, accelerating universe?

We originally defined curvature as measured by the path that light takes through the universe.Specifically, the CMBR has rippling, or fluctuations in temperature. We see hot spots and cooler spots. We can calculate what the relative sizes of the hot spots should be, based on quantum mechanical fluctuations in the kind of matter and energy field that we think existed early in the universe. If they appear magnified, compared to our theory, it indicates positive curvature (like a magnifying glass). If they appear smaller than our theory predicts, it indicates that the curvature of the universe is negative.

It turns out that the size of the ripples in the CMBR are what we would expect them to be, compared to our simulations of the early universe. This indicates a flat universe.

We can independently calculate the density of normal matter, dark matter and electromagnetic radiation. The density of the combined normal matter, dark matter and electromagnetic radiation gives a value below critical density for the universe. This would indicate a closed universe.

What we see is that the evidence indicates we have an accelerating universe. This evidence comes from the intersection of three observational methods: (1) the rippling of the CMBR, (2) the redshifts of distant galaxies using Hubble’s law and supernovae, and (3) the large scale structure seen in galaxy clusters.

Now we need to add in the density of dark energy. Our evidence indicates that something is pushing galaxies apart on a very large scale, we call it dark energy. We can calculate how much energy that would take. We can find the density by dividing the energy by volume. When we add the density of the dark energy to the density of the other three sources, we end up with a value for density that is extremely close to critical density.

But critical density no longer equates to a critically unbound universe. That is because dark energy does not act like normal matter or dark matter. It adds density without adding gravitational attraction. It appears to add density while also adding something that acts like repulsive gravity, or negative pressure.