Science Spin March 2008

The Mystery of the Missing Mass

By Chris Coughlan

In an attempt to resolve the mystery of the missing matter cosmologists have been led to conclude that there are two invisible components that permeate the universe - entities called Dark Energy and Dark Matter. The relative quantities of these two entities that are present, cosmologists believe, will determine whether the universe will, at some point in the future, start to contract and disappear, or whether it will continue to expand forever.

Relativity

Albert Einstein's original equations of general relativity implied that the fabric of space could expand or contract, and was not static or fixed. This meant that the universe might have a definite beginning, or a definite end. This defied the accepted thinking and the prevailing wisdom of the day and of Einstein himself who maintained that the universe was fixed and unchanging, and, therefore, eternal.

To overcome the possibility of a dynamic universe, Einstein modified his original equations on general relativity and introduced something called a "cosmological constant". A cosmological constant set as zero implied a changing universe, so Einstein through this mathematical trick set a nonzero constant with just the right value so as to maintain a static universe that conformed to his thinking and the thinking of the day, and brought the status of the universe back to an unchanging state.

Expanding

The wavelengths of the spectrum of light have shorter wavelengths towards the blue end and longer wavelengths towards the red end. Astronomers, by studying the electromagnetic spectrum of light emanating from stars, can deduce their temperature as well as their chemical composition.

In studying the light from distant galaxies and stars within these galaxies, astronomers found that the light spectrum shifted toward the red end of the spectrum. This effect can be considered to be the equivalent of the "Doppler Shift" of sound. For instance, when we hear a change in the pitch of sound when an ambulance moves past us at speed with its siren blaring, this is the Doppler Shift of sound.

The change in pitch of the sound from the siren is explained by the change in frequency of the sound waves moving towards our ears as the ambulance moves further away. When the ambulance is nearby the sound waves are bunched up together and hit our ears at a high frequency, so we perceive a high pitch sound. When the ambulance is further away, the sound waves are less frequent, and we perceive in these circumstances a low pitch sound.

There is a similar kind of phenomenon to explain the shift towards the red spectrum of light emanating from stars and galaxies as they move away from us. In 1929, Edwin Hubble, a US astronomer, by measuring this cosmological "Red Shift" of distant galaxies found that they were receding, or moving away from us at speeds directly proportional to their distance. He regarded this as proof that the universe was expanding.

This work by Hubble actually gave support to Einstein's original theory of general relativity, and suggested that Einstein's introduction of a cosmological constant to maintain his theory was not necessary. In the 1930s, Einstein abandoned the cosmological constant and claimed it was the greatest blunder of his scientific life.

This was not the end of this story, however, for in 1998, the concept of the cosmological constant came up again, as two independent groups of cosmologists studying supernova (exploding stars that shine brightly as they explode) in distant galaxies found that contrary to their assumption that the universe slowed as it expanded, that the reverse appeared to be true and that the expansion of the universe was in fact accelerating.

Dark

It was time for Einstein's cosmological constant to return to centre stage with a value that could be used to account for the increased accelerated expansion of the universe. A re-introduced or new cosmological constant value could be set so that it represented the energy associated with empty space. This implies that gravity changes from an attractive force to a repulsive or antigravity force that not only counteracts the gravitational attraction or pull between all objects and bodies in the universe, but also explains its acceleration.

In simple terms this force or energy uniformly fills all space and exerts a negative pressure or push that overcomes the gravitational attraction or pull. Because the force is due to space itself, the push becomes stronger over larger distances. This energy of space is now popularly referred to as "Dark Energy". Compared to the vast space of the universe the space or distance scale of our solar system is negligible and, therefore, the repulsive force or push is also negligible. This means that Newton's Laws are maintained and valid within the confines of our solar system.

Science has now reached the point where cosmologists believe that Einstein was right after all to introduce a cosmological constant, albeit for the wrong reasons and different value. Einstein's original use of the cosmological constant was to justify a static universe, while today the cosmological constant is used for practically the opposite purpose, namely to explain the expansion of the universe generally, as well as its accelerated expansion and ultimately the existence of Dark Energy.

Origins

The 'Big Bang' Model to explain the origin of the universe maintains that the universe emerged from a singularity, something of zero size and infinite compression, around 15 billion years ago. As we know the galaxies that evolved are travelling away from each other and, therefore, the universe is expanding. However, one of the fundamental questions of cosmology is, will this expansion continue indefinitely or will it stop expanding and start to contract?

The answer, scientists believe, depends on the amount of matter in the universe. More precisely, the answer depends on the critical density of matter in the universe, which in turn determines the overall spacetime geometry or shape of the universe. The critical density is calculated to be about five hydrogen atoms per cubic metre of universe.

If the critical density is exceeded the universe is "closed" and its spacetime geometry has positive curvature in a three-dimensional shape that is conceptually analogous to the surface of a sphere, which will eventually contract back to a "big crunch" as the resultant gravitational force generated across the universe will be strong enough to halt and reverse the expansion.

If the critical density is not reached then the universe is "open" and its spacetime geometry has negative curvature in a four-dimensional shape that is conceptually analogous to an infinitely extending saddle or valley between hills. The gravitational force will be too weak and the expansion will continue forever.

If the universe is neither "open" or "closed", but balanced right at the critical density and its spacetime geometry has zero curvature then it is a flat Euclidean shaped universe where the fabric of space and time will just barely expand forever.

Missing

In the 1930s, Jan Oort, the Dutch astronomer measured the Red Shifts of stars moving near the galactic plane, around the same time Fritz Zwicky of the California Institute of Technology, or Cal Tech, was studying the Galactic Coma Cluster.

The basic findings were that the stellar rotational velocity remains constant or flat with increasing distance away from the galactic centre. The rotational velocity, based on Newton's Law of Gravity, should steadily decrease for stars further away from the galactic centre. Since the galaxies were not flying apart it implied that there must be enough matter present that the gravitational pull kept the outer stars from flying away into space. Therefore, to account for these observations it was postulated that to maintain this velocity more mass would be required than was visible.

At the time this discrepancy was referred to as "missing mass". However, over time, the description "Dark Matter" replaced the term missing mass as it became apparent that the mass was present, but simply not visible. Later the anomaly was explained in the form of Dark Matter located in a spherical halo enshrouding each galaxy.

Flat

The case for a flat universe is supported by studies of the Cosmic Microwave Background, or CMB, and by "inflation theory" applied to the standard Big Bang model. The CMB is the afterglow radiation left over from the Big Bang. The CMB initially appeared equal in all directions, but in 1992 the satellite Cosmic Background Explorer (COBE) detected fluctuations in CMB temperature. Cosmologists believe this supports the thinking that the universe is spatially flat. The CMB fluctuations represent the fluctuations in the density of matter that were imprinted shortly after the Big Bang and that these can reveal a great deal about the early universe and the origin and formation of galaxies and other large objects in the universe.

Inflation theory explains the origins of fluctuations in the CMB and why the universe is balanced at the critical density. It proposes that at a time of the very early universe it underwent a period of exponential expansion which would have stretched away any possibility of large scale spatial curvature leaving the geometry of the universe flat.

In 2003 the Wilkinson Microwave Anisotropy Probe (WMAP) measured the CMB fluctuations. Cosmologists were able to determine from these measurements the basic parameters of the Big Bang and the density and composition of the universe that seemed to confirm that the universe is flat with a density break as 4 per cent visible matter, 23 per cent Dark Matter and 73 per cent Dark Energy.

What is Dark Matter?

Cosmologists have divided Dark Matter contenders into two main categories, Baryonic Dark Matter and non Baryonic Dark Matter. Baryonic is matter that we are familiar with, as it is composed of atoms or particles that are known to science. Examples of objects composed of baryonic dark matter include super massive black holes and Brown Dwarfs.

Brown Dwarfs begin life as potential stars, but there is not enough mass to initiate fusion in their cores, so they end up as non-visible bodies. Brown Dwarfs and similar Baryonic Dark Matter objects, known as Massive Compact Halo Objects, or MACHOs, are not luminous enough to be detected or directly observed by telescopes. However, they can be indirectly observed by gravitational microlensing, where the gravity of the dark invisible MACHO acts like a lens and bends light passing near it.

The problem for science is that Baryonic Dark Matter only accounts for a fraction of the total Dark Matter that is present in the universe. Therefore, some cosmologists and particle physicists have speculated that there are non-Baryonic particles and matter that remain unknown that make up the remainder of the missing Dark Matter. They call these, as yet, hypothetical particles WIMP's (Weakly Interacting Massive Particles).

Perspective

The existence of Dark Energy and Dark Matter are by no means fully accepted concepts among many cosmologists, particle physicists or astronomers. Some maintain that there are too many gaps in our knowledge, including an incomplete understanding of gravitation, to be even able to speculate. Others have put forward alternative theories of gravity, such as Modified Newtonian Dynamics, or MOND, where extra strong gravity takes the place of Dark Matter. Others still maintain that the CMB may have become distorted or corrupted over time and, therefore, the interpretations of its measurement are not reliable.

The existence of Dark Energy and Dark Matter will relegate the validity of current physics to explaining only a narrow 4 per cent of our universe. This is the equivalent of trying to determine the picture of a hundred piece jigsaw with only four pieces.

It also means that we are all, to a very large extent, perhaps not living in the real world.