Dark Matter / Dark Energy

Transcription

1 Dark Matter / Dark Energy 2 different things! The first hints of Dark Matter arose with observations of large motions in clusters of galaxies in the 1930 s. The case became compelling in the 1970 s with the discovery that the rotation curves of most galaxies are flat. The first hints of Dark Energy arose with difficulties reconciling estimates of the age of the Universe in the 1970 s and 80 s. Strong evidence came from observations of distant supernovae in 1998 that the Universe is speeding up in it s expansion. Detailed observations of the microwave background show that the Universe is topologically flat, resulting in a consistent cosmological model with important roles for both Dark Matter and Dark Energy.

2 Galaxies are embedded in halos of Dark Matter KE = PE --> mass = V 2 R/G but velocity V approx. constant with radius R so mass M increases linearly with radius R [go twice as far out -> mass is doubled] Mass increases this way to the edge of the visible galaxy --> most mass at the edge of the galaxy is dark! this is observed expected if mass follows light solar system V=1/R 1/2

3 observations of galaxy rotation slit of spectrograph superimposed on image of edge-on galaxy raw spectrum interpreted rotation curve with two halves folded together roughly flat so mass increasing linearly with R

4 sketch of dark halo visible galaxies are embedded in extended halos of dark matter

5 Dark Matter in groups and clusters of galaxies 1. Just as the motions of stars and gas within individual galaxies suggests there is unseen matter, so too does the motions of galaxies in orbit around each other in groups: virial theorem: 2 KE + PE = 0 so: M V 2 R M = mass of group; V = velocities of galaxies; R = characteristic separations typical masses 10-20x masses associated with stars and gas 2. Rich clusters contain hot intergalactic gas in equilibrium with the gravitational potential. Equilibrium temperatures give the mass creating the potential. 3. The mass in big galaxies or rich clusters can bend light --> gravitational lensing --> arcs and enhanced, multiple images Mass calculated by these 3 techniques agree --> must be massive dark halos around galaxies

6 X-ray emission from clusters of galaxies Stephan s quintet Coma cluster

7 Gravitational lensing by a cluster of galaxies

8 Bullet Cluster Contours of surface density of matter determined from weak lensing of background galaxies superimposed on an image. Same contours of the matter distribution now superimposed on a map of the distribution of X-ray emitting gas. Conclusion: Two clusters collided. The galaxies and dark matter passed through each other but the gas from each cluster collided and came to a stop. Taken as evidence for collisionless dark matter.

9 Evidence for Dark Matter on Large Scales Although the expansion of the Universe is dominant on large scales, the motions of galaxies show departures from overall expansion. - we have a `peculiar motion of 600 km/s with respect to the cosmic microwave background - all our neighbors are moving with us; ie, we are streaming in a large scale flow - we appear to be moving toward a `great attractor region 150 million light years away toward the Centaurus constellation. -rich clusters and to a lesser extent, groups and filaments are local focal points of flow patterns. The nearest big concentration is the Virgo Cluster which is pulling in galaxies over a sphere of diameter 50 million light years. -The amplitude of these flows suggest typical masses 30x greater than associated with stars

10 Need for Dark Matter to explain the growth of primordial fluctuations into galaxies: primordial fluctuation amplitudes seen in microwave background are 1 part in 105 Δρ/<ρ> 10 5 # Fluctuation amplitudes grow linearly with expansion scale in an expanding Universe until fluctuation densities become comparable with the mean density (δρ/<ρ> 1). After this point there is exponential collapse. # The Universe has only expanded by 103 since recombination. Not enough time for the growth of baryon (atomic matter) alone since this time. # Dark Matter fluctuations started growing earlier; after recombination baryons (atomic or familiar matter) fell into pre-existing Dark Matter halos.

11 Candidates for Dark Matter # The evidence for dark matter comes from the motions of things we do see. Stars in galaxies, galaxies themselves, and the gas within and between galaxies are moving around at high speeds, as they would if they are feeling strong gravitational forces. The velocities and dimensions can be measured, giving an estimate of the masses involved: M V 2 R # The sum of the evidence leads us to conclude that there is at least 10x more mass than we can account for in familiar forms. This `dark matter congregates in extended halos about visible galaxies. By and large, the interiors of galaxies are predominantly made up of matter in familiar forms [stars, interstellar gas, ] but the `dark matter is dominant at the far outer-reaches of galaxies. WIMPs, MACHOs, and dispersed gas # Dispersed gas. A reasonable possibility would be that not all matter participated in collapse into galaxies, stars, etc. A remnant could be left as intergalactic dispersed gas. However, we would be able to detect this gas unless it has very peculiar properties. It can t be very cool or very hot because we would see it directly from it s radiative properties. There is a small part of the electromagnetic spectrum in the extreme ultraviolet which is poorly studied because that radiation is absorbed by cool gas. Gas at K hide behind this veil but leave traces that would be seen if there were the huge amounts needed to solve the dark matter problem.

12 Candidates for Dark Matter # The evidence for dark matter comes from the motions of things we do see. Stars in galaxies, galaxies themselves, and the gas within and between galaxies are moving around at high speeds, as they would if they are feeling strong gravitational forces. The velocities and dimensions can be measured, giving an estimate of the masses involved: M V 2 R # The sum of the evidence leads us to conclude that there is at least 10x more mass than we can account for in familiar forms. This `dark matter congregates in extended halos about visible galaxies. By and large, the interiors of galaxies are predominantly made up of matter in familiar forms [stars, interstellar gas, ] but the `dark matter is dominant at the far outer-reaches of galaxies. WIMPs, MACHOs, and dispersed gas # Dispersed gas. A reasonable possibility would be that not all matter participated in collapse into galaxies, stars, etc. A remnant could be left as intergalactic dispersed gas. However, we would be able to detect this gas unless it has very peculiar properties. It can t be very cool or very hot because we would see it directly from it s radiative properties. There is a small part of the electromagnetic spectrum in the extreme ultraviolet which is poorly studied because that radiation is absorbed by cool gas. Gas at K hide behind this veil but leave traces that would be seen if there were the huge amounts needed to solve the dark matter problem.

13 MACHOs = MAssive Compact Halo Objects In this category would be anything that has collapsed gravitationally to a small, dense, stable configuration. Possibilities include the end points of stellar evolution: white dwarfs, neutron stars, and black holes, or low mass, slowly evolving objects like brown dwarfs. We know that objects in these categories exist! The question is whether there are the huge numbers of them that are required to solve the dark matter dilemma and, if so, how they came to have such a different distribution in space than the familiar `visible matter. Several impressive experiments have been undertaken to detect microlensing from MACHOs. The idea is to monitor the brightness of millions of stars in some background like the nearby galaxy the Large Magellanic Cloud or the central bulge of our Milky Way Galaxy. Once in awhile, a MACHO will pass in front of a background star just by chance and brighten the light of the background star by gravitational lensing. By monitoring for a long time, we derive an estimate of the density of the foreground MACHOs. Since some MACHOs surely exist, we expect to get some microlensing detections; the question is how many?

14 Gravitational Microlensing Transient magnification of background starlight (over a few weeks) by a dark object passing in front of star Monitor a few million stars to see a few dozen microlensing events per year

15 MACHOs = MAssive Compact Halo Objects In this category would be anything that has collapsed gravitationally to a small, dense, stable configuration. Possibilities include the end points of stellar evolution: white dwarfs, neutron stars, and black holes, or low mass, slowly evolving objects like brown dwarfs. We know that objects in these categories exist! The question is whether there are the huge numbers of them that are required to solve the dark matter dilemma and, if so, how they came to have such a different distribution in space than the familiar `visible matter. Several impressive experiments have been undertaken to detect microlensing from MACHOs. The idea is to monitor the brightness of millions of stars in some background like the nearby galaxy the Large Magellanic Cloud or the central bulge of our Milky Way Galaxy. Once in awhile, a MACHO will pass in front of a background star just by chance and brighten the light of the background star by gravitational lensing. By monitoring for a long time, we derive an estimate of the density of the foreground MACHOs. Since some MACHOs surely exist, we expect to get some microlensing detections; the question is how many? Results: microlensing is detected but there are not the large number of events expected if most of the dark matter is in MACHOs.

16 WIMPs = Weakly Interacting Massive Particles WIMPs are attractive candidates for the dark matter because they could easily be hung up in extended halos, just as the observations suggest. A most obvious candidate is the neutrino because we know neutrinos exist and our basic Big Bang model requires that there be huge numbers of them. Neutrinos ν are involved in transitions involving neutrons n and protons p through the Weak force; for example n + ν < > p + + e - so there were once comparable numbers of each species. The neutrinos escaped because of their low interaction probabilities and, like the cosmic microwave background radiation, should permeate space today.

17 WIMPs = Weakly Interacting Massive Particles WIMPs are attractive candidates for the dark matter because they could easily be hung up in extended halos, just as the observations suggest. A most obvious candidate is the neutrino because we know neutrinos exist and our basic Big Bang model requires that there be huge numbers of them. Neutrinos ν are involved in transitions involving neutrons n and protons p through the Weak force; for example n + ν < > p + + e - so there were once comparable numbers of each species. The neutrinos escaped because of their low interaction probabilities and, like the cosmic microwave background radiation, should permeate space today. Two problems with neutrinos as the explanation for Dark Matter: 1. They would only clump together on very large scales, because individual neutrinos are so light that they must have high motions (Hot Dark Matter), so we could only get objects like galaxies by fragmenting big structures. But galaxies seem to be a lot older than bigger structures so it is more reasonable to think that big things build out of older, littler things than the other way around. 2. Neutrinos cannot be squeezed together densely enough to form the halos detected around small galaxies. Neutrinos probably make a minor contribution to the dark matter but they almost surely are not the dominant component.

18 Several other exotic particles have been suggested as dark matter candidates; eg, axions, neutralinos, photinos, quark nuggets,. At an early stage, some such particle could have been produced abundantly, then like the neutrino, became decoupled because of a low interaction probability. If the particles have sufficient individual mass then, as the Universe ages, they would come to be moving slowly kinetic energy ~ m v 2 so through energy partition, particles with more mass m will move with lower velocities v. Particles that are moving more slowly can more easily collect together in gravitational wells. Cold Dark Matter lower velocities >> lower Jeans mass limit for collapse High mass == low velocities == Cold Dark Matter Low mass == high velocities == Hot Dark Matter (eg, neutrinos) The dominant Dark Matter particle is COLD

19 Dark Energy # Is Dark Energy related to Dark Matter? No! # Vacuum energy is a mainstream concept in physics Heisenberg uncertainty principle: ΔE Δt < h/2π h = Planck constant Energy in the vacuum can be arbitrarily large over very small time intervals. Energy can create particle - antiparticle pairs but they must subsequently annihilate in a time consistent with the uncertainty principle. `virtual particles fill the vacuum!!

20 Pressure from Vacuum Energy

21 The Casimir Effect tells us: Vacuum Energy produces Negative Pressure Why `negative? # vacuum energy pushes things apart # familiar pressure from particles acts thru gravity to pull together pull together = positive pressure push apart = negative pressure [We often think of pressure in particles as pushing apart but this is only a consequence of pressure differences. EG: we inflate a tire by introducing a high density of particles into the tire.] Negative pressure can cause space to expand!!

22 Evidence for Dark Energy Type Ia Supernovae, originating from mass accretion onto a white dwarf so that it comes to exceed the Chandrasekhar limit of 1.4 solar masses, have well defined luminosities and are so bright that they can be seen to great distances. fainter than expected Supernovae at great distances are fainter at a given redshift than expected if the Universe were being slowed by gravity or even if just coasting at a constant expansion velocity.

23 relationship between redshift and distance today Reference: Saul Perlmutter, Physics Today, April, 2003 Redshift reflects rate of expansion away from us. If Universe is slowing down then we don t have to look so far away to see a galaxy with a given redshift. A supernova in a galaxy at a given redshift that is fainter is farther away ==> Universe is speeding up.

24 In the tug-o-war between dark matter and dark energy, once dark matter was dominant and structure was collapsing but today dark energy is dominant and space is accelerating apart.