Cosmic Dawn

by George Rhee

  The Bullet cluster of galaxies shown in Fig. 4.7 illustrates the value of comparing different observations of the same object. The fact that the total mass inferred from lensing studies (blue hue) follows the visible galaxy distribution is a persuasive argument for the existence of dark matter. If one was to argue that there is no dark matter, then one would expect the lensing method to reveal a mass distribution that closely follows the X-ray gas distribution; in other words the blue and red images would overlap since the X-rays contain most of the visible matter. The fact that this is not the case and the X-ray clouds appear distorted suggests that we are observing two clusters that have passed through each other (collided) leaving the X-ray gas trailing behind the galaxies and dark matter, as one would expect from simple physical models of two colliding clusters.

  Fig. 4.7A galaxy cluster commonly known as the Bullet Cluster. The X-ray emission from this cluster is shown in red superposed on an image of the cluster taken with the Magellan Telescope in Arizona. The individual galaxies are clearly visible in the optical image. The blue hues show the distribution of the total mass of the cluster. This was mapped using gravitational lensing of background galaxies. The fact that the total mass of the cluster is dominated by an invisible component separate from the gas clouds is strong evidence for the existence of dark matter (Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.)

  Dark Matter Everywhere? The Density of the Universe

  We have discussed evidence that the universe is expanding. It is natural to speculate about the future of our universe. The universe will either keep expanding forever or recollapse into a Big Crunch in the distant future. The fate of the universe depends on whether its present density exceeds a so-called critical density. If the universe has a density greater than the critical density it will collapse at some point in the future. Future observers will see this as a change from redshift to blueshift for the light we receive from galaxies. The critical density has a value of 5 atoms per cubic meter, a number determined by the competition between the expansion energy of the universe and the force of gravity. The number measures the ratio of the true density to the critical density. If this number is greater than 1, the universe will recollapse. Let me make clear the is a ratio it is not itself a density.

  We might be tempted to say that, of course, the universe has a higher density than the critical density. After all, 5 atoms per cubic meter is much less than the best vacuum achievable on Earth. We must keep in mind, however, that we are talking of the density of the whole universe. We must average out the density of the Earth and the solar system and, indeed, our whole galaxy over vast regions of space. To get a fair measure of the density of the universe, we must sample a large region of space. I live in Boulder City, Nevada. The population density of Boulder City, 1,000 people per square mile, does not reflect the population density of the state of Nevada, which is 50 times less than that.

  How are we to estimate the density of the universe? There are a number of different methods, all of which have rather large errors. By combining these methods we can estimate the density of the universe. We can get a lower limit on the density of the universe by considering a volume of space and adding up the masses of all the galaxies within it. The density is then the mass divided by the volume. When we add up the galaxy masses, we must, of course, include the dark galaxy halos mentioned earlier in this chapter. This method yields a value for of about 0.1; that is to say, the density of the universe is 10 times less than the critical density. We can apply a similar method to clusters of galaxies. We can calculate the masses of clusters and count the number of clusters in a given volume to obtain the density of matter in that volume. This method yields values of of about 0.2. We shall discuss in a later chapter the surveys that astronomers have carried out to map the universe. These reveal that there are large regions of space containing an excess density of galaxies. These regions exert a gravitational pull on surrounding galaxies causing them to fall toward these regions. The motions the galaxies acquire are called peculiar velocities. By studying these peculiar velocities we can estimate . Such studies favor large values of , close to 1.

  We can also measure more directly by measuring the curvature of space. General relativity predicts some very strange observational effects. If we take a ruler and observe it at increasingly large distances from us, it will appear smaller and smaller. In other words, the angular size of an object gets smaller at larger distances. This is not true in cosmology however. The principle holds true at small distances, but when we get to very high redshifts of 2 and greater, objects actually appear to get bigger as the redshift increases. This is due to the way light travels through space in an expanding universe. The importance of this effect increases as the density increases. Such an effect could be used to measure . The problem is that we do not have a set of rulers placed at regular intervals in space. We only have galaxies, and they come in many different shapes and sizes. We do not know the intrinsic sizes of objects at cosmological distances from Earth.

  The apparent brightness of objects of known luminosity should also depend on . Attempts to measure the curvature of space using objects of known luminosity had been unsuccessful until a decade ago when scientists used supernovae, exploding stars, to estimate the curvature of space. The fact that supernovae occurring in binary systems all have the same luminosity suggests that they can be used as distance indicators to measure the curvature of space and hence the density of the universe. These supernovae act as light bulbs of known wattage distributed throughout space.

  Supernovae are fairly rare events. They are believed to go off at a rate of about one per 300 years in our galaxy. The technological challenge that astronomers face is thus to detect supernovae at sufficiently large redshifts and in sufficiently large numbers to measure the curvature of space. Two teams of scientist have made use of large-format imaging cameras and sophisticated detection software to carry out this ambitious project. Current results suggest , a flat universe and furthermore that there is a mysterious force at work in the universe that is countering the force of gravity and causing the expansion of the universe to speed up. This expansion force is associated with an energy known as dark energy which contributes to the energy density of the universe ().

  For over 50 years, cosmologists have dreamed of measuring the curvature of space. We must always be cautious and critical when faced with a spectacular result of this sort. We must ask ourselves what could have gone wrong, or what more mundane interpretation could account for this effect. The observations effectively state that the most distant supernovas are slightly fainter than we would expect them to be in a flat (non-curved) universe. Is there another explanation for this observation? Absorption by dust is a possibility. These issues are currently being debated in the astronomical literature. The current consensus, the result of detailed studies, is that dust cannot be responsible.

  The most accurate results are obtained by combining different scientific studies. We discuss the microwave background in some detail in the following chapter. The results of studies of the background radiation using a NASA satellite, the Wilkinson Microwave Anisotropy Probe have been combined with the supernova observations to reach the remarkable conclusion that the universe has a density of . What is remarkable about this conclusion is not only the value of but also the high precision with which this has been determined. Astronomers can now measure a number that a decade ago was known only to within a factor of ten, to a precision of about 1 %.

  What Could the Dark Matter Consist Of?

  The supernova measurements combined with the background radiation measurements can not only constrain the total density of the universe but also its different components. The value of the dark energy density responsible for the acceleration of the expansion is believed to be which means that the matter density must be about (Fig. 4.8). What could this matter consist of?
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  Fig. 4.8Relative contribution of dark matter, dark energy and atoms to the density of the universe. Ordinary atoms that we are made of contribute a relatively small amount to the total density. It turns out that about 1 % of the density of the universe is in the form of visible stars (Credit: NASA/WMAP Science Team)

  We can proceed by ruling out one candidate, the baryons. Baryons are essentially protons and neutrons, the ordinary matter that we are made of and that stars consist of. As we go back in time to the beginning of the Big Bang, the universe gets denser and denser and hotter and hotter. At the earliest times, not even atomic nuclei could have survived the intense heat. When the temperature dropped to a mere one billion Kelvin, atomic nuclei could hold together, and a process called nucleosynthesis occurred. The formation of heavier elements, such as helium and lithium, requires the formation of deuterium, an isotope of hydrogen. The deuterium nucleus consists of a proton and a neutron held together by the strong nuclear force. Initially, any deuterium nuclei that form were blasted apart by the intense heat, but when the universe cooled sufficiently, to a billion Kelvin, the deuterium nuclei could hold together.

  Let us turn our attention to the neutrons. A free neutron has a lifetime of about 15 min and will decay into a proton if left on its own. The number of free neutrons left at the time of nucleosynthesis determines the helium abundance. Along with hydrogen and helium, some other light nuclei were produced in the early universe, lithium and beryllium being of particular interest.

  Once it forms, the deuterium can then react with more protons and neutrons to form helium. It is in this way that most of the helium that we see in the universe was formed. The abundance of helium relative to hydrogen is not very sensitive to the density of the universe, it is thus a robust prediction of Big Bang theory. In other words it is difficult to imagine a universe in which little or no helium was formed early on. This result is very sensitive to the strength of the strong force that binds nuclei together. If the force were slightly stronger, all the hydrogen would have been converted to helium in the first 3 min. If the force were slightly weaker, no helium at all would have formed.

  Not all of the deuterium formed in the early universe goes to form helium. There is a little bit of deuterium left over. The abundance of deuterium is a sensitive measure of the density of baryons in the universe. In a denser universe, the reactions proceed more quickly and less deuterium will survive. It is difficult to measure the deuterium abundance because it is easily destroyed in stars. Astronomers have searched for deuterium in the atmosphere of Jupiter, in the gas between the stars, and in distant clouds of gas that absorb the light from distant quasars. They find the deuterium abundance to be very low. The baryon density () implied by the measured deuterium abundance is . The measured abundance supports this conclusion. Finally detailed studies of the microwave background variations in brightness from the WMAP satellite conclude that , that is, ordinary atoms make up only 4.6 % of the universe. Again a remarkable conclusion also remarkable for its accuracy, the previous statement is accurate to 0.1 % (Fig. 4.8).

  The baryon density today is thus about 1/25 of the critical density. However, we know that the total matter density of the universe is . The implication is that most of the dark matter in the universe is non-baryonic. The dark matter cannot consist of protons and neutrons. It must be some other form of exotic matter.

  What If the Dark Matter Consisted of Neutrinos?

  There are three lines of approach to deciding on the dominant form of dark matter. First, we can try to detect the matter through microlensing experiments. Second, we can try to detect the particles directly and attempt to measure their masses, as has been done for neutrinos. The third way is to study galaxy formation from a theoretical viewpoint and determine which form of dark matter produces galaxies that most closely resemble the ones we see today. The question of the nature of dark matter is still very much open. The most certain thing we can say (and even then, not with total certainty) is that the dark matter exists.

  One good candidate particle for dark matter is the neutrino. Neutrinos were produced in large numbers in the early universe with a density of several hundred per cubic centimeter. Given the high density of neutrinos in space, it does not take much mass for a neutrino to play a big role in cosmology. If the mass of the neutrino was about 1/20,000 of the mass of the electron, the neutrino would be the dominant form of mass in the universe. There have been several experiments to directly measure neutrino mass in the last 30 years, and none have produced convincing results. The question of whether the neutrino has mass is nevertheless resolved even though the uncertainties as to what that mass is are still very large.

  Swimming Pools Underground: Neutrino Astronomy

  There is an indirect way to prove that the neutrino has mass. There are three types of neutrinos known: the tau neutrino, the electron neutrino, and the muon neutrino. The electron, muon, and tau particles are known as leptons, particles that feel the weak interaction. If neutrinos have mass then it is possible that neutrinos could change from one kind to another. Experiments have been carried out in which a beam of muon neutrinos is produced by colliding muons with atomic nuclei. If tau leptons are detected at the point where the muons collide with the nuclei, we have evidence that muon neutrinos have changed into tau neutrinos, since only a tau neutrino can produce a tau lepton by colliding with a nucleus. There is another type of experiment currently under way that studies the particles produced by cosmic rays. The experimental setup can be thought of as a neutrino telescope. Neutrino telescopes are very different from ordinary telescopes.

  The neutrino telescope known as Super-Kamiokande, consists of a tank of pure water installed in a deep mine inside a mountain (Fig. 4.9). As the neutrinos travel through the water, they emit very weak flashes of light that are detected. The detector is direction sensitive, it can tell roughly where the neutrinos came from. The evidence is that more neutrinos are hitting the detector from above than from below. This might seem obvious, but neutrinos interact so weakly that they can travel through the whole Earth quite easily. The fact more neutrinos come from above is interpreted as meaning that some of the neutrinos coming from below are changing from one kind to another on their journey through the Earth and is thus taken as evidence of neutrino mass.

  Fig. 4.9The Super-Kamiokande neutrino detector. It consists of a stainless steel tank 39 m in diameter and 42 m tall filled with 50,000 tons of water. The detector is located 1 km under- ground in the Kamioka mine in Japan (Credit: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo)

  Neutrinos are produced by fusion reactions in the Sun. Based on the Sun’s luminosity and our understanding of nuclear fusion reactions, we can predict the rate at which neutrinos should reach the Earth. The first attempt to detect these solar neutrinos took place in the 1960s in the Homestake Mine in South Dakota. This neutrino telescope consisted of a 400,000-liter vat of cleaning fluid! The detector made use of the fact that a chlorine atom can react with a neutrino to produce an argon atom. This reaction is extremely rare–so rare that it is expected to take place only for one atom in the whole tank each day; this in spite of the fact that trillions of neutrinos pass through the tank each second. Neutrinos are very weakly interacting particles. What was intriguing and disturbing was that the Homestake experiment produced only one third as many neutrinos as was expected.

  In such a situation where a discrepancy between theory and observation arises, we have several choices. We could blame the experimentalists and claim that the measurement is incorrect. We could also claim that the theoretical predictions are wrong; in other words, we don’t really understand the inner workings of the Sun. One way to keep both theorists and experimenters happy is to challenge neither. We can argue instead that both the prediction and the measurement are correct, but that the solar neutrinos (electron neutrinos) are turning into tau and muon neutrinos on the way from the Sun to the Earth. Since the Homestake experiment
of the 1960s, three experiments have come online that confirm the original experimental results. The problem, however, is not quite as bad. Instead of only 30 % of the predicted neutrinos being detected 70 % of the neutrinos are now being observed. Maybe we should wait another 30 years to find the remaining third of the missing neutrinos!

  The current trend is to argue in favor of neutrinos transforming from one kind to another and to argue that this is evidence for neutrino mass. The limits on the mass of the neutrino are not reliable, but it is still possible that neutrinos could account for a substantial amount of the dark matter in the universe. Neutrinos aret also thought to be emitted in very large amounts during supernova explosions. When a supernova was detected in a companion galaxy to ours in 1987, two neutrino detectors were operational and detected a burst of neutrinos coming from that direction. This was a remarkable confirmation of astrophysical theories of supernova explosions. The supernova was visible only in the southern hemisphere, yet the neutrino detectors were in the northern hemisphere so the neutrinos passed through most of the Earth before being detected.

  Neutrinos are a form of dark matter known as hot dark matter. They move close to the speed of light in the early universe. This has consequences for galaxy formation which we will discuss in a later chapter. A form of matter that has proven very useful in constructing models of galaxy formation is known as cold dark matter. These particles move around slowly in the early universe. Although they are very popular with theoretical astrophysicists, they have probably not been directly detected on Earth. There are experiments deep underground that are designed to detect these cold dark matter candidates. The search for dark matter particles highlights a theme of cosmology; the link between the very large and the very small. We will see that the properties of elementary particles determine the nature of the largest structures in the universe.