Nature Outlook |
The dark universe
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.
For more on the dark universe from nature.com, see: nature.com/subjects/particle-astrophysics
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The leading theory of dark matter is running out of room to hide.
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.
The path to understanding dark energy begins with a single question: has it always been the same throughout the history of the Universe?
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.
Particle physicists have proposed many different solutions to the dark matter problem: dark matter constitutes 85% of all the matter in the Universe, but we still don't know what it actually is. Of the competing candidates, WIMPs (weakly interacting massive particles) are attracting particular attention because they arise naturally from new theories that seek to extend the standard model of particle physics. In a review in this issue of Nature, Gianfranco Bertone explains why he thinks the moment of truth has arrived for WIMPs. If the Large Hadron Collider at CERN, Europe's particle-physics lab near Geneva, Switzerland, and the latest astroparticle experiments fail to observe WIMPs in the next decade, he reckons, the new era of physics will be on hold for 20 years or more. By that time, a new generation of particle colliders might have arrived.
In a Letter to Nature in October 1979, Charles Alcock and Bohdan Paczynski (
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.
The ΛCDM or Lambda-CDM model has been regarded as the standard model of cosmology for decade or so, as it explains many of the most important observations. The model assumes that the 95% of mass and energy of the Universe not accounted for in the visible Universe is present as dark energy (Λ) and cold dark matter (CDM). Where it does fall down, though, is in its apparent inability to explain the low-density 'cores' of dark matter measured at the centres of galaxies. Historically, the effects of normal matter on the dark matter have been ignored. In this review, Andrew Pontzen and Fabio Governato point to recent work that shows how gas and stars can significantly alter the effect of CDM through a coupling based on rapid gravitational potential fluctuations.
Established cosmological models of galaxy formation and evolution have achieved limited success, failing to create the mixed population of elliptical and spiral galaxies that we observe. A new simulation that makes full use of the latest advances in computing power and algorithmic developments successfully recreates a population of ellipticals and spirals, reproduces the observed distribution of galaxies in clusters, the evolution of dark and visible matter and the characteristics of hydrogen on large scales, at the same time matching the metal (heavier than helium) and hydrogen content of galaxies on small scales. The calculation tracks the build-up of galaxies at unprecedented precision from shortly after the Big Bang until the present day, spanning more than 13 billion years of cosmic evolution.
The relativistic Big Bang theory of cosmic evolution gives a good description of our expanding Universe on the grand scale. But closer to home, where we can observe galactic properties in detail, its predictions go awry. For instance, some of the largest galaxies in our neighbourhood are found in less crowded regions, contrary to standard-model predictions. And the region known as the Local Void contains many fewer galaxies than expected. The observations of nearby galaxies are more understandable if it is assumed that matter forms more rapidly into galaxies and clusters than current theory allows. Jim Peebles and Adi Nusser outline recent efforts by cosmologists to adapt fundamental theory to let new physics operate on the scale of galaxies, yet preserve the properties of the present model on cosmological scales.