The easy part is over experimental cosmology has now reached adulthood. That was the message that emerged from a workshop on cosmology and particle physics held at CERN this summer.
A continuing challenge is to find this missing matter material we cannot see but which has to be there to explain the gravitational behaviour we do see.
While cosmology is one of the oldest of sciences, it is only this century that it has become truly quantitative, with measurements from ground-based detectors extending beyond the traditional visible window and, more recently, with data from an impressive array of space-borne instrumentation. Underlining the new maturity of the science are the emerging values for the basic parameters of the cosmological equations.
A latter-day Copernican revolution came when Edwin Hubble discovered in the 1920s that the universe is still expanding, subsequently understood to be the aftermath of the initial Big Bang. Ever since, observational cosmology has tried to pin down how this expansion has evolved. The “Hubble constant” the apparent ratio between expansion velocity and distance has long been controversial. One typical “result” was the paradox that the universe appeared to be younger than its oldest stars the “old wine in new bottles” dilemma.
Thanks to new data, including parallax measurements from the Hipparcos satellite, the Hubble constant and the age of far-flung objects in the universe are now more compatible. The oldest stars are of the same vintage as our universe.
Talks at the CERN meeting, covering observations from the Hubble Space Telescope and other satellites and from systematic supernova searches, showed that the “world average” Hubble constant now looks to be about 67, with a likely age of the universe about 14 gigayears.
Wendy Freedman of the Hubble Space Telescope team showed that the spectrum of the Hubble flow looks remarkably smooth (with the local “infall” drift towards Virgo subtracted). With reliable new data, statistical fluctuations have largely gone away, and the emphasis turns instead to systematic effects.
Observations of distant supernovae, which exploded when the universe was still young, reveal how the universe has since expanded. For the supernova search teams, Saul Perlmutter and Robert Kirshner demonstrated how the subtle effects now being seen at these extreme distances cannot be fitted by a single Hubble constant, and the idea of a “cosmological constant” an anti-gravity repulsion has made a comeback.
According to the basic Big Bang/Hubble picture, the further away an object is, the faster it appears to recede, with the expansion of the universe inexorably slowing as gravity steadily applies the brakes. However, the data from supernovae suggest this is an oversimplified picture, with an anti-gravity effect assisting the expansion, so that the Big Bang can sometimes appear to accelerate.
This reopens the debate on whether the universe is “open”, continuing to expand for ever, or “closed”, ultimately to disappear in a final “Big Crunch”. Neither is yet excluded.
At the CERN workshop, inflation pioneer Andrei Linde showed how an infant universe born in a quantum fluctuation supposedly attained its present proportions due to a brief initial flash of “inflation” which transformed a quantum bubble into a living universe. The incredible rate of this explosion strongly suggests total reconciliation with gravity, so that what we now see should be “flat”, neither continually expanding nor destined to recombine.
Achieving a flat universe with the new cosmological data is not ruled out, but the cosmological constant plays an important role. Flatness is not achieved by conventional gravitational pull alone.
Although inflation practically dictates a flat universe, there is not enough visible matter out there to accomplish the task, and invisible “dark matter” is invoked to provide the extra gravitational pull needed to close the universe. A continuing challenge is to find this missing matter material we cannot see but which has to be there to explain the gravitational behaviour we do see. However, the arrival of a non-zero cosmological constant provides an additional gravitational effect to help close the universe using less dark matter.
One dark matter candidate is MACHOS Massive Astrophysical Compact Halo Objects. At the CERN workshop, Michel Spiro summarized the search for MACHOs using gravitational lensing, in which otherwise invisible intervening matter can affect the image of more distant objects as they move across the sky.
One MACHO-seeking collaboration, itself called MACHO, now has 14 candidates in the direction of the Large Magellanic Cloud (LMC), whose durations range from 15 to 90 days. Another collaboration EROS has two, each lasting about four weeks. MACHO covers most of the LMC, but with low efficiency, while the complementary EROS search covers a restricted area containing some 150,000 stars with high efficiency. Taken together, these results imply that planetary mass objects account for less than 10% of the halo. Their attention is now also extended to the Small Magellanic Cloud, while other dark-matter searches have also joined the hunt.