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We know the existence of dark matter because we can test its gravitational mass (e.g. in gravitational lens) but, since we cannot see this matter, how we can be sure that it has an inertial mass, and it is the same as the gravitational mass? In other words, does the principle of equivalence apply also to dark matter?
If the inertial mass is not equal to the gravitational mass, it would be equivalent to the gravitational constant, G, being different, or perhaps non-constant, for dark matter. If that were so, the manner in which dark matter would orbit the milky way would be different, leading to a different distribution of dark matter
We know from the rotation curve of the milky way roughly how dark matter is distributed in the milky way. Our models are consistent with the inertial mass of dark matter being equal to the gravitational mass.
This is, of course, far from "proof". As we don't even know what dark matter is, it is impossible to be certain of any of its properties. And the equivalence of inertial and graviational mass is not "proved" even for normal matter (but no experiment has ever detected a difference) But it would be exceedingly surprising if gravity affected dark matter differently, and there would need to be strong evidence for that. As it stands, there is none.
Dark matter inertial mass - Astronomy
The existence of visible and dark matter, and the continued direct undetectability of dark matter, necessitates a theoretical investigation into the nature of inertial mass that constitutes visible and dark matter. It is proposed that inertial mass is manifested dually: materially, as extrinsic inertial mass, and immaterially, as intrinsic inertial mass. Visible matter experiences inertia when accelerated because the applied real force on the material extrinsic manifestation of inertial mass is not cancelled by the equal and opposite inertial pseudo force on the immaterial intrinsic manifestation of inertial mass. It is also proposed that all visible matter experience inertia all the time in cosmological acceleration, because spacetime real force on extrinsic inertial mass is not cancelled by the inertial pseudo force on intrinsic inertial mass. Visible matter in gravitational freefall does not experiencing inertia because inertial pseudo force associated with intrinsic inertial mass is cancelled by the pseudo-like gravitational force associated with gravitational mass. Im-material intrinsic inertial mass also exists independently as intrinsic inertial mass objects, which are in cosmological freefall and do not experience inertia, because of cancellation between spacetime pseudo-like force and inertial pseudo force on intrinsic inertial mass. Immaterial intrinsic inertial mass objects in cosmological freefall are the dark matter in the universe.
The Case for Dark Matter
The evidence for dark matter lies with gravity. Gravity is the force or "glue" that holds the universe together. Everything in the universe is mutually attracted to everything else. Scientists have been able to calculate the total mass of the visible universe. They have also calculated the gravitational forces that hold the universe together. What they have found is that there does not appear to be enough visible matter to account for the mass that is required to gravitationally bind the universe together. In addition, dark matter can be detected through its gravitational influence on other objects, or even on light itself. It can affect the motion of stars and galaxies. Many galaxies have been found to be rotating much faster that they should. According to Einstein's theory of gravity, they should fly apart. But something unseen seems to be holding them together. Dark matter can also affect the path of light. In a phenomenon known as gravitational lensing, dense objects can cause the light of distant objects to bend around it. This can result in distorted images and duplicate images of stars and galaxies. We can tell that something is bending the light, but we can't tell what it is.
Astronomy Picture of the Day Index - Miscellaneous: Dark Matter
APOD: 2001 December 19 - Finding Dark Matter
Explanation: Where is dark matter? Galaxies rotate and move in clusters as if a tremendous amount of unseen matter is present. But does dark matter exist in the greater universe too -- and if so, where? The answer can be found by comparing the distribution of galaxies observed with numerical simulations. This comparison became much more accurate recently when over 100,000 galaxy observations from the 2-Degree Field Galactic Redshift Survey were used. In the above frame from a computer simulation of our universe, a 300 million light-year slice shows dark matter in gray and galaxies as colored circles. The red box indicates the location of a rich cluster of galaxies, while the green box shows a more typical cross-section of our universe. Analyses indicate that the immense gravity of the pervasive dark matter pulls normal matter to it, so that light matter and dark matter actually cluster together.
APOD: 2003 August 14 - Dark Matter Map
Explanation: The total mass within giant galaxy cluster CL0025+1654, about 4.5 billion light-years away, produces a cosmic gravitational lens -- bending light as predicted by Einstein's theory of gravity and forming detectable images of even more distant background galaxies. Of course, the total cluster mass is the sum of the galaxies themselves, seen as ordinary luminous matter, plus the cluster's invisible dark matter whose nature remains unknown. But by analyzing the distribution of luminous matter and the properties of the gravitational lensing due to total cluster mass, researchers have solved the problem of tracing the dark matter layout. Their resulting map shows the otherwise invisible dark matter in blue, and the positions of the cluster galaxies in yellow. The work, based on extensive Hubble Space Telescope observations, reveals that the cluster's dark matter is not evenly distributed, but follows the clumps of luminous matter closely.
APOD: 2005 September 25 - WMAP Resolves the Universe
Explanation: Analyses of a new high-resolution map of microwave light emitted only 380,000 years after the Big Bang appear to define our universe more precisely than ever before. The eagerly awaited results announced last year from the orbiting Wilkinson Microwave Anisotropy Probe resolve several long-standing disagreements in cosmology rooted in less precise data. Specifically, present analyses of above WMAP all-sky image indicate that the universe is 13.7 billion years old (accurate to 1 percent), composed of 73 percent dark energy, 23 percent cold dark matter, and only 4 percent atoms, is currently expanding at the rate of 71 km/sec/Mpc (accurate to 5 percent), underwent episodes of rapid expansion called inflation, and will expand forever.
Dark Matter Bound to our Solar System
Dark matter might become captured inside of our solar system, bound to the Earth or to the Sun. If so, we may be closer to discovering dark matter than previously believed.
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Dark matter constitutes the majority of the total mass of our Milky Way galaxy, 10x more than all the luminous matter (stars and gas) that we see in the night sky. Indirect observation suggests that dark matter exists here, right here, at the position of the Earth. It passes through our bodies every second it is literally right under our noses. Yet, this mysterious substance has until now evaded detection by the world's most sensitive instruments.
Why is dark matter so difficult to find? For at least three reasons. First, we know any interactions it has with 'ordinary' matter must be exceedingly feeble therefore any experimental search must be that much more sensitive to weak signals. Second, there are many models for the constituent particles that make up the dark matter we simply don't know, yet, what their mass might be, what (feeble) interactions they might have, or what their cosmological history might have been. Thus we need different experiments to probe different sets of models.
Our research focuses on an interesting candidate called a relaxion, which interacts with ordinary matter in a way that is very similar to the Higgs boson. While the Higgs boson gives a constant mass to the electron, the relaxion field in our galaxy would oscillate and induce fluctuations in the electron's mass. This in turn modifies electron orbits and can be searched for in table-top experiments that measure, for example, the energy levels in atomic transitions. This is a unique and striking signal. but difficult to probe. The current reach of experiments is truly impressive depending on the relaxion mass, current sensitivity allows searches for oscillations at the level of 1 part in 10 19 (!). Yet, because the relaxion is so weakly coupled, these experiments have not been able to probe the model of relaxion dark matter.
Relaxion dark matter would induce oscillations in the Higgs field, which induce oscillations in the mass of electrons, in turn modifying atomic orbitals.
. or, maybe they have? This brings me to the third reason that dark matter is so difficult to find. You see, the usual idea for dark matter in the galaxy is that it exists in a large spherical halo, which is dense in the center and less so outside. In this model, at our position in the galaxy (about 26,000 light years from the galactic center) the density is only 10 -21 kg/m 3 , less than 0.000000000000000000001 of the density of water. So while dark matter is indeed here, right here, under the usual assumption it is extremely dilute. No wonder the signals are so weak!
The standard picture of the large-scale galactic dark matter halo (size and position of solar system not to scale).
In our work, we asked the question: what if this assumption isn't correct? What if in addition to the large-scale halo, relaxion dark matter can be captured inside of our solar system, bound to the large gravitational potential of the Earth or the Sun? These bound clumps, which we call relaxion halos, would have the structure of gravitational atoms, with the Earth or Sun acting as the nucleus. Just like ordinary atoms, the radius of the 'orbit' depends inversely on the mass of the relaxion, so the relaxion Earth halo and the relaxion solar halo actually can be searched for independently. The scenario is constrained by observations of the motion of planets, the moon, and satellites in low Earth orbit, but without violating any constraint it is possible that a large density of dark matter is bound in such objects, as much as 10 10 times higher density than in the standard scenario.
Two possible scenarios for Relaxion Halos: those bound to the Sun (left) and Earth (right).
If the relaxion Earth halo is larger than the Earth's radius, or if the relaxion solar halo is larger than 1 astronomical unit (AU), then terrestrial experiments will find themselves inside of a relaxion halo. The experimental signal would then see a huge enhancement relative the usual assumption of dilute background dark matter. This scenario, if it is realized in our solar system, implies that current and near-future atomic physics experiments will soon either discover relaxion dark matter, or rule it out over a wide range of parameters.
This scenario may not be as wild as it sounds at first. We are investigating mechanisms for the formation of relaxion halos, both in the context of ordinary gravitational relaxation (which is known to produce the free-floating clumps of relaxion known as boson stars), and also through new dissipation mechanisms. Also, importantly, such halos could exist for other similar dark matter candidates, like axions and axion-like particles, modifying the sensitivity of many other experimental searches. If we do indeed live inside of a relaxion halo, we may be closer to discovering dark matter than we thought!
Dark matter inertial mass - Astronomy
DAMIC100 installation at SNOLAB
A packaged CCD being inserted in the copper box. Above the box is a lead cylinder shielding the CCDs from radiogenic backgrounds. DAMIC100 comprises 18 CCDs for a total mass of 100 g.
Click on the image to enlarge
The DAMIC (Dark Matter In CCDs) experiment employs a new technique for searching for the elusive particles that we think make up most of the matter in the universe -- dark matter.
DAMIC uses charged coupled devices -- the CCDs that have been used for many years in digital cameras, but these are not your average CCDs. They are the high-tech ones also used in the Dark Energy Camera, which Fermilab installed on the Blanco telescope in Chile. The detectors were developed and fabricated at Berkeley Lab and were tested and installed in the camera at Fermilab. They are unusually thick (250 microns instead of the usual 30) and have low intrinsic noise levels, making them ideal for the long exposure times needed to search for the rare interactions expected for dark-matter particles.
Dark matter inertial mass - Astronomy
The first clue to the existence of Dark Matter is studying the rotation curve of our Milky Way galaxy. Kepler's laws or rotation (which hold well for planetary orbits around our Sun) states that stars in the outer spiral arm should rotate much slower than stars toward the center of the galaxy but they don't:
Other evidence that Dark Matter is a real phenomenon:
- The halo of a galaxy contain more mass than can be seen visually - determined by gravitation effects on the host galaxy
- Hot X-ray gas found in clusters have remained due to unseen gravitational forces
- Distortion effects of distant quasars - the gravity lens
- Light from distant galaxies demonstrate hydrogen absorption lines from unseen matter
- Motions of galaxy clusters themselves indicate some strong gravity influence from unseen sources
I wrote a project paper on Dark Matter found in the advanced topics section that goes into so detail about determining possible sources of Dark Matter. Basically the two competing theories are:
Black holes, brown dwarfs, white dwarfs and other massive objects only equal a small percentage of unseen matter. Hot dark matter is thought to be near zero-mass moving near the speed of light. This can be relativistic moving massive neutrinos. Cold dark matter compose of more massive particles moving slower than the speed of light.
One interesting effect of Dark Matter surrounding a galaxy is that objects that lay behind the galaxy in the line of sight of an observer will witness what is called a gravity lens. The mass of the galaxy itself is not enough to affect the light from the distant object, but Dark Matter can.
(Image credit: Brooks/Cole Thomson Learning)
A dramatic lens effect is seen with the nearby cluster of galaxies - Abell 2218:
Computer simulations are very important in determining the role of Dark Matter and the formation of the structure of our Universe. As discussed earlier, galaxies exist in clusters and clusters are members of superclusters. All of this is held together by Dark Matter.
The problem that astronomers face now is attempting to determine if Dark Matter is Cold, Hot or both. There are two major theories as to the structure of our Universe, and each depends on either Hot or Cold Dark Matter.
- The Top Down model - Hot Dark Matter dominated Universe break apart to form clusters of galaxies
- The Bottom Up model - A Cold Dark Matter dominated Universe starts out with small clumps that coalesce into larger clusters of galaxies
Data is still being collected and debates are sometimes very heated.
Dark Matter Candidates from Particle Physics and Methods of Detection
The identity of dark matter is a question of central importance in both astrophysics and particle physics. In the past, the leading particle candidates were cold and collisionless, and typically predicted missing energy signals at particle colliders. However, recent progress has greatly expanded the list of well-motivated candidates and the possible signatures of dark matter. This review begins with a brief summary of the standard model of particle physics and its outstanding problems. I then discuss several dark matter candidates motivated by these problems, including weakly interacting massive particles (WIMPs), superWIMPs, light gravitinos, hidden dark matter, sterile neutrinos, and axions. For each of these, I critically examine the particle physics motivations and present their expected production mechanisms, basic properties, and implications for direct and indirect detection, particle colliders, and astrophysical observations. Upcoming experiments will discover or exclude many of these candidates, and progress may open up an era of unprecedented synergy between studies of the largest and smallest observable length scales.
In astronomy and cosmology, dark matter is a type of matter hypothesized to account for a large part of the total mass in the universe. Dark matter cannot be seen directly with telescopes evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level. Instead, its existence and properties are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. According to the Planck mission team, and based on the standard model of cosmology, the total mass&ndashenergy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe.
Dark matter came to the attention of astrophysicists due to discrepancies between the mass of large astronomical objects determined from their gravitational effects, and the mass calculated from the "luminous matter" they contain: stars, gas and dust. It was first postulated by Jan Oort in 1932 to account for the orbital velocities of stars in the Milky Way, and by Fritz Zwicky in 1933 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, many other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxies by Vera Rubin, in the 1960s-1970s, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, the temperature distribution of hot gas in galaxies and clusters of galaxies, and more recently the pattern of anisotropies in the cosmic microwave background. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle. The search for this particle, by a variety of means, is one of the major efforts in particle physics today.
Although the existence of dark matter is generally accepted by the mainstream scientific community, there is no generally agreed direct detection of it. Other theories including MOND and TeVeS, are some alternative theories of gravity proposed to try to explain the anomalies for which dark matter is intended to account.
On 3 April 2013, NASA scientists reported that hints of dark matter may have been detected by the Alpha Magnetic Spectrometer on the International Space Station. According to the scientists, "The first results from the space-borne Alpha Magnetic Spectrometer confirm an unexplained excess of high-energy positrons in Earth-bound cosmic rays."
Dark matter inertial mass - Astronomy
Key points: Evidence for dark matter ideas for what it is Evidence for Dark Energy
|If the mass followed the "normal" matter -- stars and gas -- the rotation speed would drop like the "Keplerian motion" line, like for the planets. Then their speeds would be as we derived when we were discussing Kepler's Laws. This relation assumes essentially all the mass is in the central object (the sun for the planetary system).||Instead, the rotation curve is nearly flat with increasing radius. Evidently there are huge amounts of unseen "dark" matter in the outer parts of the galaxy that add gravitational field beyond that just from the center, causing the stars and gas to orbit faster. (Figures from The Essential Cosmic Perspective, by Bennett et al.)|
|Like the Milky Way, virtually all galaxies have flat rotation curves to well beyond where they have many stars, indicating that they are all surrounded by large halos of dark matter. (From The Essential Cosmic Perspective, by Bennett et al.)|
When we account carefully for the mass in stars in a galaxy, it turns out to be much less than the mass we measure from Newton's laws! In addition, there appears to be mass we can't see outside the region occupied by the stars. As much of 90% of galaxies may be in some form of unseen mass.
We have no good idea of what galaxies are mostly made!! Is there some basic particle of physics that we don't know about that accounts for the unseen mass? This is evidently the dark matter we know played such a central role in shaping the Universe, but all we know about local examples is from galaxy rotation curves. A good link for further information is at http://www.eclipse.net/
Thus, the distance measurements using Type 1 supernovae indicate that the expansion of the Universe is getting faster.
Brian Schmidt at the Nobel Prize ceremony
It is humbling, perhaps even humiliating, that we know almost nothing about 96% of what is "out there"!!
Test your understanding before going on
In the 18th Century, Thomas Wright proposed that theUniverse was filled with groupings of stars like the Milky Way, from