|Evidence for the expanding universe
Measuring the mass of galaxies
We have seen how Hubble used Doppler shift to determine the recessional velocity of galaxies. However, by using Doppler shift in a slightly different way, scientists can learn much about how galaxies move. They know that galaxies rotate because, when viewed edge-on, the light from one side of the galaxy is blue-shifted and the light from the other side is red-shifted. One side is moving towards the Earth, the other is moving away. The speed at which the galaxy is rotating can also be calculated from how far the light is shifted. Knowing how fast the galaxy is rotating, the mass of the galaxy can be found mathematically.
When scientists looked closer at the speeds of galactic rotation, they found something strange. Classical physics would determine that the individual stars in a galaxy should act similarly to the planets in our solar system – the greater the distance from the centre, the slower they should move. But the results from Doppler shift measurements reveal that the stars in many galaxies do not slow down at farther distances. In fact, the stars move at speeds that should see them escape the galaxy’s gravitational field; there is not enough measured mass to supply the gravity needed to hold the galaxy together. (See: http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/imagine/act_modeling.html.)
This would suggest that a galaxy with such high rotational speeds in its stars contains more mass than is predicted by calculations. Scientists theorise that, if the galaxy was surrounded by a halo of unseen matter, the galaxy could remain stable at such high rotational speeds.
You can have a go at weighing the Milky Way at the following website: http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/imagine/student_weighing.html.
Seeing the light
Astronomers can also use measurements of how much light there is to determine the mass of a galaxy (or a cluster of galaxies). By measuring the amount of light reaching the Earth, scientists can estimate the number of stars in the galaxy. Knowing the number of stars in the galaxy, scientists can mathematically determine the mass of the galaxy.
Fritz Zwicky used both methods described here to determine the mass of the Coma cluster of galaxies over half a century ago. Using our second technique, Zwicky estimated the total mass of a group of galaxies by measuring their brightness. But when the other method was used to compute the mass of the same cluster of galaxies, his calculations came up with a number that was 400 times greater than his original estimate. The discrepancy in the observed and computed masses is now known as ‘the missing mass problem’. The high rotational speeds that suggest a halo reinforce Zwicky’s findings. Zwicky’s findings were little used until the 1970s, when scientists began to realise that only large amounts of hidden mass could explain many of their observations. Scientists also realised that the existence of some unseen mass would also support theories regarding the structure of the universe. Today, scientists are searching for the mysterious dark matter, not only to explain the gravitational motions of galaxies but also to validate current theories about the origin and the fate of the universe.
Repeatedly using different methods to establish the masses of galaxies has found discrepancies that suggest that approximately 90% of the universe is matter in a form that cannot be seen, known as dark matter. Some scientists think dark matter is in the form of massive objects, such as black holes, that are situated around unseen galaxies. Other scientists believe dark matter to be subatomic particles that rarely interact with ordinary matter.
Dark matter is the term given to matter that does not appear to be emitting electromagnetic radiation, i.e. matter that cannot be seen. Scientists can infer that the dark matter is there from observations of its effects, but they cannot directly view it. Bruce H. Margon, chairman of the astronomy department at the University of Washington, told the New York Times, ‘It’s a fairly embarrassing situation to admit that we can’t find 90 per cent of the universe’. This problem has scientists scrambling to try and find where and what this dark matter is. ‘What it is is any body’s guess,’ adds Dr Margon. ‘Mother Nature is having a double laugh. She’s hidden most of the matter in the universe, and hidden it in a form that can’t be seen’.
Examine the evidence for the missing matter here:
Look at an explanation of how we can search for dark matter here:
MACHOs vs. WIMPs
So how do we look for dark matter? It can’t be seen or touched: we know of its existence by implication. It has been speculated that dark matter could be anything from tiny subatomic particles having 100,000 times less mass than an electron to black holes with masses millions of times that of the Sun. This has divided scientists into two schools of thought as they consider possible candidates for dark matter. These have been dubbed MACHOs (massive astrophysical compact halo objects) and WIMPs (weakly interacting massive particles). Although these acronyms are amusing, they can help you remember which is which. MACHOs are the big, strong dark matter objects ranging in size from small stars to super massive black holes. MACHOs are made of ‘ordinary’ matter, which is called baryonic matter. WIMPs, however, are small weak subatomic dark matter candidates, and are thought to be made of material other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs. (Baryonic matter is made up of hydrogen and helium atoms, non-baryonic is made up of subatomic particles.)
Astronomers and particle physicists disagree about what they think dark matter is. Walter Stockwell, of the dark matter team at the Center for Particle Astrophysics at University of California at Berkeley, describes this difference:
The nature of what we find to be the dark matter will have a great effect on particle physics and astronomy. The controversy starts when people made theories of what this matter could be and the first split is between ordinary baryonic matter and non-baryonic matter.
Since MACHOs are too far away and WIMPs are too small to be seen, astronomers and particle physicists have devised ways of trying to infer their existence.
Massive compact halo objects are non-luminous objects that make up the halos around galaxies as suggested by Zwicky. In the first instance MACHOs are thought to be brown dwarf stars or black holes. Their existence was predicted by theory long before there was any proof. The existence of brown dwarfs was predicted by theories that describe star formation. Albert Einstein’s general theory of relativity famously predicted black holes, but the idea was first suggested by John Michell based on Newton’s corpuscular theory of light.
Brown dwarfs, like the Sun, are made from hydrogen, but they are usually much smaller. Stars like our Sun form when a mass of hydrogen collapses under its own gravity and the intense pressure initiates a nuclear reaction, emitting light and energy. Brown dwarfs differ from normal stars insofar as because of their relatively low mass they do not have enough gravity to ignite when they form. A brown dwarf therefore does not become a ‘real’ star; it is merely an accumulation of hydrogen gas held together by gravity. Brown dwarfs do give off some heat and a small amount of light.
Black holes, unlike brown dwarfs, are created by an overabundance of matter. A star made of hydrogen forms helium when the hydrogen atoms collide in the star. Eventually, the hydrogen fuel is used up and the gas starts to cool. All that matter ‘collapses’ under its own enormous gravity into a relatively small area. The black hole is so dense that anything that comes too close to it, even light, cannot escape the pull of its gravitational field. Stars at a safe distance, beyond the event horizon, will circle around the black hole, much like the motion of the planets around the Sun. It was first believed that black holes emitted no light – that they were truly black. However, it is now believed that high-energy particles are ejected in jets along the axis of rotation of a black hole.
Astronomers are faced with quite a challenge in detecting MACHOs. They must detect, over astronomical distances, things that give off little or no light. However, the task is becoming easier as astronomers create more refined telescopes and techniques for detecting MACHOs.
Searching with Hubble
Using the Hubble Space Telescope, astronomers can detect brown dwarfs in the halos of our own and nearby galaxies. However, the images produced do not reveal the large numbers of brown dwarfs that astronomers hoped to find. ‘We expected [the Hubble images] to be covered wall to wall by faint, red stars,’ reported Francesco Paresce of the Johns Hopkins University Space Telescope Science Institute in the Chronicle of Higher Education. Research results disappointed: calculations based on the Hubble research estimate that brown dwarfs constitute only 6% of galactic halo matter.
Astronomers use a technique called gravitational lensing in the search for dark matter halo objects. Gravitational lensing occurs when a massive dark object passes between a light source, such as a star or a galaxy, and an observer on the Earth. The gravitational field is so large that it causes the
light to bend. The object focuses the light rays, causing the intensity of the light source to apparently increase. Astronomers diligently search photographs of the night sky for the telltale brightening that indicates the presence of a MACHO.
So why doesn’t a MACHO block the light? How can dark matter act like a lens? The answer is gravity. Albert Einstein proved in 1919 that gravity bends light rays. He predicted that a star that was positioned behind the Sun would be visible during a total eclipse. Einstein was correct: the Sun’s gravitational field bent the light rays coming from the star and made it appear next to the sun.
Not only can astronomers detect MACHOs with the gravitational lens technique, but they can also calculate the mass of the MACHO by determining distances and the duration of the lens effect. Although gravitational lensing has been known since Einstein’s demonstration, astronomers have only begun to use the technique to look for MACHOs in the past 20 years.
Gravitational lensing projects include the MACHO project (America and Australia), the EROS project (France) and the OGLE project (America and Poland).
Another way to detect a black hole is to notice the gravitational effect that it has on objects around it. When astronomers see stars circling around a gravitational mass, but cannot see what that mass is, they suspect a black hole. Then by carefully observing the circling objects, the astronomers can conclude that a black hole does exist.
In January 1995, a team of American and Japanese scientists announced ‘compelling evidence’ for the existence of a massive black hole at the American Astronomical Society meeting. Led by Dr Makoto Miyosi of the Mizusawa Astrogeodynamics Observatory and Dr James Moran of the Harvard-Smithsonian Center for Astrophysics, this group calculated the rotational velocity from the Doppler shifts of circling stars to determine the mass of a black hole. This black hole has a mass equivalent to 36 million times that of our sun. While this finding and others like it are encouraging, MACHO researchers have not turned up enough brown dwarfs and black holes to account for the missing mass. Thus most scientists concede that dark matter is a combination of baryonic MACHOs and non-baryonic WIMPs.
Evidence for MACHOs: Astronomers have observed objects that are either brown dwarfs or large planets around other stars using the properties of gravitational lenses.
Evidence against MACHOs: While they have been observed, astronomers have found no evidence of a large enough population of brown dwarfs that would account for all the dark matter in our Galaxy.
In their efforts to find the missing 90% of the universe, particle physicists theorise about the existence of tiny non-baryonic particles that are different from what we call ‘ordinary’ matter. The prime candidates include neutrinos, axions and neutralinos. Subatomic WIMPs are thought to have mass (whether they are light or heavy depends on the particle), but usually only interact with baryonic matter gravitationally; they pass right through ordinary matter. Since each WIMP has only a small amount of mass, there needs to be a large number of them to account for the missing matter. That means that millions of WIMPs are passing through ordinary matter, the Earth and you and me, every few seconds. Although some people claim that WIMPs were proposed only because they provide a ‘quick fix’ to the missing matter problem, neutrinos were first ‘invented’ by physicists in the early 20th century to help make particle physics interactions work properly. They were later observed, and physicists and astronomers now have a good idea how many neutrinos there are in the universe. However, they are thought to be without mass – in 1998 one type of neutrino was discovered to have a mass, but it was insufficient for the neutrinos to contribute significantly to dark matter.
According to Walter Stockwell, astronomers also concede that at least some of the missing matter must be WIMPs. ‘I think the MACHO groups themselves would tell you that they can’t say MACHOs make up the dark matter’. The problem with searching for WIMPs is that they rarely interact with ordinary matter, which makes them difficult to detect.
Axions are particles which have been proposed to explain the absence of an electrical dipole moment for the neutron. They thus serve a purpose for both particle physics and for astronomy. Although axions may not have much mass, they would have been produced abundantly in the Big Bang. Current searches for axions include laboratory experiments and searches in the halo of our galaxy and in the Sun.
Neutralinos are members of another set of particles that has been proposed as part of a physics theory known as supersymmetry. This theory is one that attempts to unify all the known forces in physics. Neutralinos are massive particles (they may be 30–5000 times the mass of the proton), but they are the lightest of the electrically neutral supersymmetric particles. Astronomers and physicists are developing ways of detecting the neutralino, either underground or searching the universe for signs of their interactions.
Evidence for WIMPS: Theoretically, there is the possibility that very massive subatomic particles, created in the right numbers and with the right properties in the first moments of time after the Big Bang, are the dark matter of the universe. These particles are also important to physicists who seek to understand the nature of subatomic physics.
Evidence against WIMPS: The neutrino does not have enough mass to be a major component of dark matter. Observations have so far not detected axions or neutralinos.
There are other factors that help scientists determine the mix between MACHOs and WIMPs as components of the dark matter. Recent results by the WMAP satellite show that our universe is made up of only 4% ordinary matter. This seems to exclude a large component of MACHOs. About 23% of our universe is dark matter. This favours the dark matter being made up mostly of some type of WIMP. However, the evolution of structure in the universe indicates that the dark matter must not be fast moving, since fast-moving particles prevent the clumping of matter in the universe. So while neutrinos may make up part of the dark matter, they are not a major component. Particles such as the axion and neutralino appear to have the appropriate properties to be dark matter, but they have yet to be detected.
Debate the arguments
Evidence against the expanding universe
OUR DYNAMIC UNIVERSE (H, PHYSICS)
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