Gravity's dark side

By Aussiegirl

The "dark" in my title refers to the elusive "dark matter" that has so far eluded cosmology. Here is Wikipedia's article on dark matter -- the opening paragraph describes the problem: In cosmology, dark matter refers to matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence may be inferred from gravitational effects on visible matter such as stars and galaxies. Dark matter explains several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter also resolves a number of seeming inconsistencies in the Big Bang theory, and is crucial for structure formation. It looks like a big battle is shaping up between the upstarts and the comfortably established. But then, as the last paragraph states:
It is still too early to tell whether we are indeed witnessing such a scientific revolution. But in the mean time, we should bear in mind that some of the fiercest resistance to alternative theories of gravity was once directed at Einstein and Newton themselves.


Gravity's dark side (June 2006) - Physics World - PhysicsWeb

Despite decades of searching, the "dark matter" thought to hold galaxies together is still nowhere to be found. Matthew Chalmers describes how some physicists think it makes more sense to change our theory of gravity instead. [....]
.
The same cannot be said of a growing number of professional physicists who think Einstein's general theory of relativity is ripe for revision. General relativity is part of the bedrock of modern physics. It describes in elegant mathematical terms how matter causes space-time to curve, and therefore how objects move in a gravitational field. Since it was published in 1916, general relativity has passed every test asked of it with flying colours, and to many physicists the notion that it is wrong is sacrilege.

But the motivation for developing an alternative theory of gravity is compelling. Over the last few years cosmologists have arrived at a simple yet extraordinarily successful model of universe. The trouble is that it requires most of the cosmos to be filled with mysterious stuff that we cannot see. In particular, general relativity - or rather its non-relativistic limit otherwise known as Newtonian gravity - can only correctly describe the dynamics of galaxies if we invoke huge quantities of "dark matter". Furthermore, an exotic entity called dark energy is necessary to account for the recent discovery that the expansion of the universe is accelerating. Indeed, in the standard model of cosmology, visible matter such as stars, planets and physics textbooks accounts for just 4% of the total universe.

Faced with a lack of direct evidence for dark matter, a small but growing band of physicists is proposing an alternative explanation: that our description of gravity is wrong. And if the discussions at a recent workshop on dark matter and alternative gravities held in April at the Royal Observatory in Edinburgh are anything to go by, they could be in for a bumpy ride. [....]

Dark matter was proposed in 1933 to explain why galaxies in certain clusters move faster than would be possible if they contained only the "baryonic" matter that we can see. A few decades later, similar behaviour was detected in individual galaxies, whereby the rotational velocity of the outermost stars was found not to "drop off" as a function of distance but instead remain flat (see figure). These observations directly contradicted Newtonian gravity, which should hold true in extragalactic regions just as it does on Earth and in the solar system. But by assuming there are "haloes" of invisible matter in and around galactic structures, Newton's familiar inverse square law is restored.

Although firmly embedded in modern cosmology, dark matter is viewed by many physicists as a fudge factor. "Astronomers have no idea what dark matter is," says HongSheng Zhao of St Andrews University. "It is whatever is needed to explain the data, rather than a fundamental prediction of particle physics as it was originally." The situation is reminiscent of one facing astronomers in the 1840s, who in trying to explain anomalies in the orbit of Uranus postulated a new outer planet rather than scrap Newton's law. The crucial difference, of course, is that Neptune was discovered shortly afterwards, while dark matter remains elusive despite years of dedicated searches. [....]

Cosmology is perhaps the toughest challenge for alternative gravity theories. In particular, a theory of gravity needs to explain how structures such as galaxies and galaxy clusters formed. And this means it has to correctly describe the cosmic microwave background - the ancient radiation that comes from a period called recombination that occurred about 380,000 years after the Big Bang.

Before recombination the universe was a hot, dense plasma in which photons were continuously being scattered by charged particles. But as soon as the universe cooled enough for neutral atoms to form, photons were able to travel unhindered though space, thereby carrying vital information about density irregularities in the primordial plasma. Today, these irregularities, which would have caused matter to clump together in some regions more than others, appear as hot and cold patches in the faint glow of microwave radiation that permeates the deep universe.

General relativity is very good at describing how cosmic structure evolved from these acoustic oscillations, but only if we assume the universe contained at least as much dark matter as it did baryonic matter during recombination. Without dark matter, which couples to matter but not to light, the density fluctuations would have been smoothed out by collisions with photons long before they had a chance to seed galaxies. [....]

In addition to fitting galaxy data very well, Moffat claims that his theory can also explain the wayward paths of the Pioneer probes. Launched in the 1970s these two spacecraft, which are now at the furthest reaches of the solar system, appear to be experiencing an anomalous acceleration towards the inner solar system. Although this may turn out to have a mundane technical explanation, it provides a classic test for alternative theories of gravity on solar-system scales.

While describing the universe without dark matter is the main goal of STVG and other alternative gravity theories, it would be nice if such theories could get rid of dark energy at the same time. General relativity has a chequered history in this regard. Einstein initially introduced a constant term to account for the then-observational fact that the universe was static, only to have to remove it again a few years later when Hubble discovered that the universe is expanding. Einstein called the cosmological constant his "biggest blunder". But had he been around in 1997 to see supernova data that revealed that the expansion of the universe is actually accelerating, he would have had to put it back in again - only to be told that its value was out by some 120 orders of magnitude! [....]

Anyone working on alternative theories of gravity has to justify their ideas to mainstream cosmologists, most of whom simply do not see why there should be alternatives to dark matter. "General relativity and dark matter are the standard paradigm, and until we see something inconsistent with that then there is no reason to look elsewhere," insists Robert Caldwell of Dartmouth College in the US.

Some astronomers, such as John Peacock of the Royal Observatory, also remind us that standard Newtonian-Einstein gravity works very well from millimetre scales to the orbit of Pluto, so people should be cautious before writing it off "based on messy astrophysics such as galaxy formation". Indeed, one astronomer at the Edinburgh meeting even said he only attended to see alternative gravity theories "shot to pieces".

But many in the alternative-gravity camp think that the idea of dark matter and dark energy has become so embedded that people are no longer looking at the problem scientifically. According to Stacy McGaugh of the University of Maryland, who became sceptical about dark matter in the mid-1990s, the best way to face up to the problem is to actually work with the galaxy data. "If all the famous cosmologists who are currently defending the standard model had been deep frozen in the 1970s or 1980s and then you woke them up today and said [dark matter and dark energy] is the answer, not one of them would buy it," he jokes.

But the alternative-gravity camp also has problems of its own. For example, while TeVeS is the most advanced theory on the table, some researchers - notably Moffat and Mannheim - point out that the theory contains preferred frames of reference that violate basic relativity principles. Then there is the issue of aesthetics. Is plucking terms from the air and putting them into the action of general relativity any more respectable than fitting the galaxy data with a polynomial or some other function? Finally, dark matter could turn up tomorrow in one of the many dedicated searches world wide.

As well as being a rich ground for theoretical physics, the alternative-gravity movement will doubtless prove equally fertile for sociologists of science. "This is a potential paradigm shift in physics," says Moffat. "You will always face opposition when you attempt to modify a well accepted theory such as general relativity."

It is still too early to tell whether we are indeed witnessing such a scientific revolution. But in the mean time, we should bear in mind that some of the fiercest resistance to alternative theories of gravity was once directed at Einstein and Newton themselves.