Archive for September, 2006

Instant Expert: Cosmology

(Image: NASA)

(Image: NASA)

Cosmologists study the universe as a whole: its birth, growth, shape, size and eventual fate. The vast scale of the universe became clear in the 1920s when Edwin Hubble proved that “spiral nebulae” are actually other galaxies like ours, millions to billions of light years away.

Hubble found that most galaxies are red shifted: the spectrum of their light is moved to longer, redder wavelengths. This can be explained as a doppler shift if the galaxies are moving away from us. Fainter, more distant galaxies have higher red shift, implying that they are receding faster, in a relationship set by the hubble constant.

The discovery that the whole universe is expanding led to the big bang theory. This states that if everything is flying apart now, it was once presumably packed much closer together, in a hot dense state. A rival idea, the steady-state theory, holds that new matter is constantly being created to fill the gaps generated by expansion. But the big bang largely triumphed in 1965 when Arno Penzias and Robert Wilson discovered cosmic microwave background radiation. This is relic heat radiation emitted by hot matter in the very early universe, 380,000 years after the first instant of the big bang.

Space-time curve

The growth of the universe can be modelled with Albert Einstein’s general theory of relativity, which desribes how matter and energy make space-time curve. We feel that curvature as the force of gravity. Assuming the cosmological principle (that on the largest scales the universe is uniform), general relativity produces fairly simple equations called Friedmann models to describe how space curves and expands.

According to these models, the shape of the universe could be like the surface of a sphere, or curved like the surface of a saddle. But in fact, observations suggest that it is poised between the two, almost exactly flat. One explanation is the theory of inflation. This states that during the first split second of existence, space expanded at terrifying speed, flattening out any original curvature. Then today’s observable universe, grew from a microscopic patch of the original fireball. This would also explain the horizon problem – why it is that one side of the universe is almost the same density and temperature as the other.

The universe is not totally smooth, however, and in 1990 the COBE satellite detected ripples in the cosmic microwave background, the signature of primordial density fluctuations. These slight ripples in the early universe may have been generated by random quantum fluctuations in the energy field that drove inflation. Topological defects in space could also have caused the fluctuations, but they do not fit the pattern well.

Those density fluctuations form the seeds of galaxies and galaxy clusters, which are scattered throughout the universe with a foamy large-scale structure on scales of up to about a billion light years. All these structures form because gravity amplifies the original fluctuations, pulling denser patches of matter together.

Dark matter

In simulations, however, visible matter does not supply enough gravity to create the structure we see: it has to be helped out by some form of dark matter. More evidence for the dark stuff comes from galaxies that are rotating too fast to hold together without extra gravitational glue.

Dark matter can’t be like ordinary matter, because it would have made too much deuterium in big-bang nucleosynthesis. When the universe was less than 3 minutes old, some protons and neutrons fused to make light elements, and cosmologists calculate that if there had been much more ordinary matter than we see, then the dense cauldron would have brewed up a lot more deuterium than is observed.

Instead, dark matter must be something exotic, probably generated in the hot early moments of the big bang – maybe particles such as WIMPs (weakly interacting massive particles) or lighter axions, or, less likely, primordial black holes. An alternative to dark matter is modified Newtonian dynamics, or MOND, a theory in which gravity is relatively strong at long range.

Dark energy

Another dark mystery emerged in the 1990s, when astronomers found that distant supernovae are surprisingly faint – suggesting that the expansion of the universe is not slowing down as everyone expected, but accelerating. The universe seems to be dominated by some repulsive force, or antigravity, which has been dubbed dark energy. It may be a cosmological constant (or vacuum energy) or a changing energy field such as quintessence. It could stem from the strange properties of neutrinos, or it could be another modification of gravity.

The WMAP spacecraft put the standard picture of cosmology on a firm footing by precisely measuring the spectrum of fluctuations in the microwave background, which fits a universe 13.7 billion years old, containing 4% ordinary matter, 22% dark matter, and 74% dark energy. WMAP’s picture also fits inflationary theory. However, a sterner test of inflation awaits the detection of cosmic gravitational waves, which the rapid motions of inflation ought to create, and which would leave subtle marks on the microwave background.

The density of dark energy is far smaller than the vacuum energy predicted by quantum theory. That is seen as an extreme example of cosmological fine tuning, in that a much larger value would have torn apart gathering gas clouds and prevented any stars from forming. That has led some cosmologists to adopt the anthropic principle – that the properties of our universe have to be suited for life, otherwise we would not be here to observe it.

Unanswered questions

The biggest questions are still unanswered. We do not know the true size of the universe, even whether it is infinite or not. Nor do we know its topology – whether space wraps around on itself. We do not know what caused inflation, or whether it has created a plethora of parallel universes far from our own, as many inflationary theories imply.

And it is not clear why the universe favours matter over antimatter. Early in the big bang, when particles were being created, there must have been a strong bias towards matter, which the standard model of particle physics cannot explain. Otherwise matter and antimatter would have annihilated each other and there would be almost nothing left but radiation.

The fate of the universe depends on the unknown nature of dark energy and how it behaves in the future: galaxies might become isolated by acceleration, or all matter could be destroyed in a big rip, or the universe might collapse in a big crunch – perhaps re-expanding as a cyclic universe. The universe could even be swallowed by a giant wormhole.

And the true beginning, if there was one, is still unknown, because at the initial singularity all known physical theories break down. To understand the origin of the universe we will probably need a theory of quantum gravity.

Top 10: Weirdest cosmology theories

Cosmology is one of the most creative and bizarre areas of science. Explore some of the strangest ideas in this exclusive feature

Could space-time actually be a

Could space-time actually be a “superfluid” substance, scattered with whirling vortices? (Image: Forex/Rex Features)

1. Clashing branes

Could our universe be a membrane floating in higher dimensional space, repeatedly smashing into a neighbouring universe? According to an offshoot of string theory called braneworld, there are large extra dimensions of space, and while gravity can reach out into them, we are confined to our own “brane” universe with only three dimensions. Neil Turok of Cambridge University in the UK and Paul Steinhardt of Princeton University in New Jersey, US, have worked out how the big bang could have been sparked when our universe clashed violently with another. These clashes repeat, producing a new big bang every now and then – so if the cyclic universe model is right, the cosmos could be immortal.

2. Evolving universes

When matter is compressed to extreme densities at the centre of a black hole, it might bounce back and create a new baby universe. The laws of physics in the offspring might differ slightly, and at random, from the parent – so universes might evolve, suggests Lee Smolin of the Perimeter Institute in Waterloo, Canada. Universes that make a lot of black holes have a lot of children, so eventually they come to dominate the population of the multiverse. If we live in a typical universe, then it ought to have physical laws and constants that optimise the production of black holes. It is not yet known whether our universe fits the bill.

3. Superfluid space-time

One of the most outlandish new theories of cosmology is that space-time is actually a superfluid substance, flowing with zero friction. Then if the universe is rotating, superfluid spacetime would be scattered with vortices, according to physicists Pawel Mazur of the University of South Carolina and George Chapline at Lawrence Livermore lab in California – and those vortices might have seeded structures such as galaxies. Mazur suggests that our universe might have been born in a collapsing star, where the combination of stellar matter and superfluid space could spawn dark energy, the repulsive force that is accelerating the expansion of the universe.

4. Goldilocks universe

Why does the universe have properties that are “just right” to permit the emergence of life? Tinker with a few physical constants and we would end up with no stars, or no matter, or a universe that lasts only for the blink of an eye. One answer is the anthropic principle: the universe we see has to be hospitable, or we would not be here to observe it. Recently the idea has gained some strength, because the theory of inflation suggests that there may be an infinity of universes out there, and string theory hints that they might have an almost infinite range of different properties and physical laws. But many cosmologists dismiss the anthropic principle as being non-science, because it makes no testable predictions.

5. Gravity reaches out

Dark matter might not really be “stuff” – it could just be a misleading name for the odd behaviour of gravity. The theory called MOND (modified Newtonian dynamics), suggests that gravity does not fade away as quickly as current theories predict. This stronger gravity can fill the role of dark matter, holding together galaxies and clusters that would otherwise fly apart. A new formulation of MOND, consistent with relativity, has rekindled interest in the idea, although it may not fit the pattern of spots in the cosmic microwave background.

6. Cosmic ghost

Three mysteries of modern cosmology could be wrapped up in one ghostly presence. After making an adjustment to Einstein’s general theory of relativity, a team of physicists found a strange substance popping out of their new theory, the “ghost condensate“. It can produce repulsive gravity to drive cosmic inflation in the big bang, while later on it could generate the more sedate acceleration that is ascribed to dark energy. Moreover, if this slippery substance clumps together, it could form dark matter.

7. It’s a small universe

The pattern of spots in the cosmic microwave background has a suspicious deficiency: there are surprisingly few big spots. One possible explanation is that the universe is small – so small that, back when the microwave background was being produced, it just could not hold those big blobs. If so, space would have to wrap around on itself somehow. Possibly the oddest suggestion is that the universe is funnel-shaped, with one narrow end and one flared end like the bell of a trumpet. The bent-back curvature of space in this model would also stretch out any smaller microwave spots from round blobs into the little ellipses that are indeed observed.

8. Fast light

Why do opposite sides of the universe look the same? It’s a puzzle because the extremes of today’s visible universe should never have been in touch. Even back in the early moments of the big bang, when these areas were much closer together, there wasn’t enough time for light – or anything else – to travel from one to another. There was no time for temperature and density to get evened out; and yet they are even. One solution: light used to move much faster. But to make that work could mean a radical overhaul of Einstein’s theory of relativity.

9. Sterile neutrinos

Dark matter might be made of the most elusive particles ever imagined – sterile neutrinos. They are hypothetical heavier cousins of ordinary neutrinos and would interact with other matter only through the force of gravity – making them essentially impossible to detect. But they might have the right properties to be “warm” dark matter, buzzing about at speeds of a few kilometres per second, forming the largish dark matter clumps mapped by recent observations. Sterile neutrinos could also help stars and black holes to form in the early universe, and give the kicks that send neutron stars speeding around our galaxy.

10. In the Matrix

Maybe our universe isn’t real. Philosopher Nick Bostrom has claimed that we are probably living inside a computer simulation. Assuming it ever becomes possible to simulate consciousness, then presumably future civilisations would try it, probably many times over. Most perceived universes would be simulated ones – so chances are we are in one of them. In that case, perhaps all those cosmological oddities such as dark matter and dark energy are simply patches, stuck on to cover up early inconsistencies in our simulation.

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