Cosmology
The origin, contents and history of the universe as a whole.
arXiv: astro-ph.CO
12 topics
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The cosmic microwave background is a nearly uniform glow from when the universe was 380,000 years old, with tiny hot and cold spots. On the largest angles those spots show patterns (missing correlations, alignments, a lopsided sky) that the standard model rates as unlikely, and the distribution of distant galaxies on the sky does not quite match our motion inferred from the glow.
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Between about 100 million and 1 billion years after the Big Bang the first stars, galaxies and black holes formed and their light ionized the hydrogen gas. The James Webb Space Telescope (JWST) finds more bright galaxies and large black holes at these times than expected, and radio telescopes try to detect the 21-cm radio line of hydrogen from this era.
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Origin of cosmic magnetic fields
4 problemsGalaxies and galaxy clusters carry magnetic fields of a few millionths of a gauss, and gamma-ray observations suggest that even the near-empty voids between galaxies are weakly magnetized. Nobody knows whether these fields started from seeds made in the early universe or later by stars and galaxies.
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Dark energy and the cosmological constant
6 problemsThe expansion of the universe has been speeding up for several billion years, as if empty space carries an energy that pushes outward. Simple estimates from quantum theory give a vacuum energy enormously larger than the value observed, and recent surveys hint that the dark energy may change with time.
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Particle identity of dark matter
4 problemsAbout 85 percent of the matter in the universe neither emits nor absorbs light and is detected only through its gravity. Nobody knows what it is made of: a new elementary particle, a very light wave-like field, or black holes formed in the early universe.
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Simulations of cold dark matter predict how mass is arranged inside galaxies, and some observations of small galaxies disagree in detail. Galaxies also obey tight empirical rules linking their visible matter to their total gravity, rules that MOND (modified Newtonian dynamics, a change of gravity at very low accelerations) builds in from the start.
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The present expansion rate of the universe, the Hubble constant, comes out about 8 percent higher from nearby stars and supernovae than from the cosmic microwave background (the leftover glow of the early universe) interpreted with the standard model. The two values disagree by more than five times their combined uncertainty.
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Inflation and primordial perturbations
4 problemsInflation is a proposed burst of extremely fast expansion in the first tiny fraction of a second that would explain why the universe is so uniform and flat and would seed galaxies from quantum fluctuations. It has passed several tests but is not confirmed, and the field that drove it is unknown.
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In the first few minutes the universe fused hydrogen into helium and small amounts of deuterium and lithium, and theory predicts the amounts precisely. Deuterium and helium roughly match observations, but the oldest stars contain about three times less lithium-7 than predicted.
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Primordial black holes
5 problemsBlack holes might have formed in the first second of the universe from rare, very dense regions, with masses anywhere from a mountain to many suns. Finding even one would reveal density ripples of the early universe on scales far smaller than any other probe reaches.
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The cosmic beginning and quantum cosmology
4 problemsRunning the expansion backward leads to a moment where ordinary physics stops working, and nobody knows whether time had a beginning or the universe bounced from an earlier phase. Quantum theory applied to the whole universe is the main tool, and its basic rules for making predictions are still disputed.
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Growth of structure and the neutrino mass
6 problemsMatter clumps into galaxies and clusters over time, and how fast it clumps depends on the ingredients of the universe, including the small masses of neutrinos (nearly massless particles that barely interact). Some surveys see less clumping than predicted, and the cosmological limit on neutrino mass now presses against the minimum allowed by laboratory experiments.