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Category: cosmology – Page 75

A physicist investigating black holes has found that, in an expanding universe, Einstein’s equations require that the rate of the universe’s expansion at the event horizon of every black hole must be a constant, the same for all black holes. In turn this means that the only energy at the event horizon is dark energy, the so-called cosmological constant. The study is published on the arXiv preprint server.

“Otherwise,” said Nikodem Popławski, a Distinguished Lecturer at the University of New Haven, “the pressure of matter and curvature of spacetime would have to be infinite at a horizon, but that is unphysical.”

Black holes are a fascinating topic because they are about the simplest things in the universe: their only properties are mass, electric charge and angular momentum (spin). Yet their simplicity gives rise to a fantastical property—they have an event horizon at a critical distance from the black hole, a nonphysical surface around it, spherical in the simplest cases. Anything closer to the black hole, that is, inside the event horizon, can never escape the black hole.

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Dark energy—a mysterious force pushing the universe apart at an ever-increasing rate—was discovered 26 years ago, and ever since, scientists have been searching for a new and exotic particle causing the expansion.

Pushing the boundaries of this search, University of California, Berkeley physicists have now built the most precise experiment yet to look for minor deviations from the accepted theory of that could be evidence for such a particle, which theorists have dubbed a chameleon or symmetron. The results are published in the June 11, 2024, issue of Nature Physics.

The experiment, which combines an for precise gravity measurements with an to hold the atoms in place, allowed the researchers to immobilize free-falling atoms for seconds instead of milliseconds to look for gravitational effects, besting the current most precise measurement by a factor of five.

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New studies suggest the Nancy Grace Roman Space Telescope could detect primordial black holes from the early universe, potentially confirming their role in cosmic inflation and as components of dark matter.

When astrophysicists observe the cosmos, they see different types of black holes. They range from gargantuan supermassive black holes with billions of solar masses to difficult-to-find intermediate-mass black holes (IMBHs) all the way down to smaller stellar-mass black holes.

But there may be another class of these objects: primordial black holes (PBHs) that formed in the very early Universe. If they exist, the Nancy Grace Roman Space Telescope should be able to spot them.

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The study suggests these primordial black holes could have absorbed free quarks and gluons, making them different from typical black holes formed by collapsing stars. They would be incredibly small, yet could account for much of the universe’s dark matter.

For decades, scientists have struggled to explain the lack of visible matter in the universe.

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Domain walls, long a divisive topic in physics, may be ideal explanations for some bizarre cosmic quirks.

By Anil Ananthaswamy

“As long as they live for long enough, they will always become large cosmological beasts,” says Ricardo Ferreira, a cosmologist at the University of Coimbra in Portugal. He’s not talking about actual beasts but rather about hypothetical humongous sheets of spacetime that could divide one region of the universe from another. Such so-called domain walls are the natural outcome of theories that try to solve some of the deepest mysteries in physics, such as the origins of gravity. As Ferreira says, however, had they formed after the big bang, by today they’d be the dominant source of energy in our universe, and there’s no evidence that’s the case. So any theory invoking their existence has been considered suspect—until now, perhaps.

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The standard theory of cosmology—which says 95% of the universe is made up of unknown stuff we can’t see—has passed its strictest test yet. The first results released from an instrument designed to study the cosmic effects of mysterious dark energy confirm that, to the nearest 1%, the universe has evolved over the past 11 billion years just as theorists have predicted.

The findings, presented today in a series of talks at the American Physical Society meeting in Sacramento, California, and the Moriond meeting in Italy, as well as in a set of preprints posted to arXiv, come from the Dark Energy Spectroscopic Instrument (DESI), which has logged more than 6 million galaxies in deep space to construct the largest 3D map of the universe yet compiled.

“It’s a tremendous instrument and a major result,” says Eric Gawiser, a cosmologist at Rutgers University who was not involved with the work. “The universe DESI is finding is very sensible, with tantalizing hints of a more interesting one.”

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After its “birth” in the Big Bang, the universe consisted mainly of hydrogen and a few helium atoms. These are the lightest elements in the periodic table. More-or-less all elements heavier than helium were produced in the 13.8 billion years between the Big Bang and the present day.

Stars have produced many of these heavier elements through the process of nuclear fusion. However, this only makes elements as heavy as iron. The creation of any heavier elements would consume energy instead of releasing it.

In order to explain the presence of these heavier elements today, it’s necessary to find phenomena that can produce them. One type of event that fits the bill is a gamma-ray burst (GRB)—the most powerful class of explosion in the universe. These can erupt with a quintillion (10 followed by 18 zeros) times the luminosity of our sun, and are thought to be caused by several types of event.

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