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It’s a sobering thought: all the matter that has ever been detected accounts for a mere 4.9% of the Universe. Most of the cosmos is the dark universe, a mix of dark matter and dark energy. Both have so far proved impenetrable puzzles, but physicists young and old are intent on changing that.
Astronomy is entering an era in which gravitational waves and neutrinos will be used to complement existing techniques and to uncover the hidden features of our Universe.
George Smoot shared the 2006 Nobel Prize in Physics for the discovery of small temperature variations in the cosmic microwave background radiation, providing support for Big Bang theory. Smoot spoke to Nature about last year's big cosmological discovery, gravitational waves.
In 1998, Brian Schmidt discovered that, contrary to expectations, the expansion of the Universe is accelerating. The discovery won him a share of the 2011 Nobel Prize in Physics and launched the search to uncover the nature of dark energy.
Scientists have theories about dark matter and dark energy — and some observations — but both are poorly understood. Here are four of their biggest questions.
The Universe is expanding. And the expansion seems to be speeding up. To account for that acceleration, a mysterious factor, 'dark energy', is often invoked. A contrary opinion — that this factor isn't at all mysterious — is here given voice, along with counter-arguments against that view.
Neutrinos from deep space can be used as astronomical messengers, providing clues about the origin of cosmic rays or dark matter. The IceCube experiment is leading the way in neutrino astronomy.
We know that dark matter constitutes 85 per cent of all the matter in the Universe, but we do not know of what it is made. Among the many dark matter candidates proposed, WIMPs (weakly interacting massive particles) occupy a special place. The moment of truth has now come for WIMPs: either we will discover them in the next five to ten years, or we will witness their inevitable decline.
The acceleration of the expansion of the Universe is attributed to a 'dark energy' component that opposes gravity. These authors report an analysis of the symmetry properties of distant pairs of galaxies from archival data. This allows them to determine that the Universe is flat, and by alternately fixing its spatial geometry and the dark energy equation-of-state parameter, wX, they establish at the 68.3 per cent confidence level that −0.85 > wX > −1.12 and 0.60 < ΩX < 0.80, where ΩX is the abundance of dark energy.
A cosmological model treating dark matter as a coherent quantum wave agrees well with conventional dark-matter theory on an astronomical scale. But on smaller scales, the quantum nature of wave-like dark matter can explain dark-matter cores that are observed in dwarf galaxies, which standard theory cannot.
In the ΛCDM paradigm, 95% of the Universe consists of dark energy and cold dark matter, but the low-density cores of dark matter measured at the centre of galaxies are hard to explain using this model; here a review of recent work shows that the action of stars and gas can significantly alter the distribution of cold dark matter through a coupling based on rapid gravitational potential fluctuations.
A simulation that starts 12 million years after the Big Bang and traces 13 billion years of cosmic evolution yields a reasonable population of elliptical and spiral galaxies, reproduces the observed distribution of galaxies in clusters and the characteristics of hydrogen on large scales, and at the same time matches the ‘metal’ and hydrogen content of galaxies on small scales.
The relativistic Big Bang theory is a good description of our expanding Universe. But — as discussed in this review article — a still better theory would describe a mechanism by which matter is more rapidly gathered into galaxies and groups of galaxies, better fitting the observations.