Dec 3, 2022 08:38 PM
Is dark matter’s “nightmare scenario” true?
https://bigthink.com/starts-with-a-bang/...-scenario/
KEY TAKEAWAYS: Ever since the need for some source of gravitation, beyond the normal matter particles we know of, became apparent, dark matter has become the leading explanation for our observations. While the indirect, astrophysical evidence supporting its existence is overwhelming, all direct detection efforts have come up empty. This isn't evidence against dark matter's existence, but it may be evidence that the "nightmare scenario" of dark matter that only interacts gravitationally, might be true... (MORE - details)
General relativity may need tweaking on the grand scale of the Universe
https://www.advancedsciencenews.com/gene...-universe/
EXCERPTS: Einstein’s theory of general relativity, formulated in 1916, treats gravity as a deformation of spacetime as a result of different particles and fields. Together with the Standard Model describing electromagnetic, weak and strong forces, it constitutes the basis of our modern understanding of particle physics.
Despite the remarkable success of this theory in explaining gravitational effects on the scales of planets, stars, and galaxies, general relativity still has its limitations. It has been known for around a century that it doesn’t explain quantum effects, which in the case of gravity are expected to become important at the scale approximately 20 orders of magnitude smaller than the size of an atom. This means that to describe the physics of gravity at such extremely small distances, one cannot use general relativity.
However, recent astronomical observations indicate that on the largest of scales — that of the entire Universe — general relativity may also lose its validity.
[...] the 1998 discovery of the accelerated expansion of the Universe forced scientists to introduce another ingredient into the theory called dark energy, which is sometimes interpreted as the energy of the vacuum, and according to Einstein’s theory, causes the expansion.
For general relativity to be consistent with this astronomical data, these two entities—dark matter and dark energy—must account for about 95% of all energy in the Universe. However, neither have manifested themselves in any laboratory experiments, and their origin and properties do not have a reliable theoretical explanation, which has led some scientists to think that they may not exist at all.
Even more difficulties arose after the discovery of what astronomers called the Hubble tension, in which the values of the expansion rate of the Universe measured in different experiments contradict one another. For example, in observations made of supernova explosions and the properties of the cosmic microwave background — radiation left over from the Big Bang.
“The discrepancy does not automatically mean that general relativity is incorrect or that something else is wrong with our cosmological model,” said Pogosian in an e-mail. “We have to keep an open mind and allow for something yet unaccounted for in the data analysis. However, this possibility has been (and continues to be) under tight scrutiny and, so far, the tension only got stronger with further analysis.”
Nevertheless, many cosmologists wonder if the contradictions between theory and observation should be resolved by a modification of the theory of gravity. Possible improvements of general relativity have been studied by researchers for decades, but the most popular of them is a set of theories proposed in 1974 by Gregory Horndeski of the University of Waterloo, Canada, in which the details of how matter curves spacetime may differ from Einstein’s theory... (MORE - missing details)
https://bigthink.com/starts-with-a-bang/...-scenario/
KEY TAKEAWAYS: Ever since the need for some source of gravitation, beyond the normal matter particles we know of, became apparent, dark matter has become the leading explanation for our observations. While the indirect, astrophysical evidence supporting its existence is overwhelming, all direct detection efforts have come up empty. This isn't evidence against dark matter's existence, but it may be evidence that the "nightmare scenario" of dark matter that only interacts gravitationally, might be true... (MORE - details)
General relativity may need tweaking on the grand scale of the Universe
https://www.advancedsciencenews.com/gene...-universe/
EXCERPTS: Einstein’s theory of general relativity, formulated in 1916, treats gravity as a deformation of spacetime as a result of different particles and fields. Together with the Standard Model describing electromagnetic, weak and strong forces, it constitutes the basis of our modern understanding of particle physics.
Despite the remarkable success of this theory in explaining gravitational effects on the scales of planets, stars, and galaxies, general relativity still has its limitations. It has been known for around a century that it doesn’t explain quantum effects, which in the case of gravity are expected to become important at the scale approximately 20 orders of magnitude smaller than the size of an atom. This means that to describe the physics of gravity at such extremely small distances, one cannot use general relativity.
However, recent astronomical observations indicate that on the largest of scales — that of the entire Universe — general relativity may also lose its validity.
[...] the 1998 discovery of the accelerated expansion of the Universe forced scientists to introduce another ingredient into the theory called dark energy, which is sometimes interpreted as the energy of the vacuum, and according to Einstein’s theory, causes the expansion.
For general relativity to be consistent with this astronomical data, these two entities—dark matter and dark energy—must account for about 95% of all energy in the Universe. However, neither have manifested themselves in any laboratory experiments, and their origin and properties do not have a reliable theoretical explanation, which has led some scientists to think that they may not exist at all.
Even more difficulties arose after the discovery of what astronomers called the Hubble tension, in which the values of the expansion rate of the Universe measured in different experiments contradict one another. For example, in observations made of supernova explosions and the properties of the cosmic microwave background — radiation left over from the Big Bang.
“The discrepancy does not automatically mean that general relativity is incorrect or that something else is wrong with our cosmological model,” said Pogosian in an e-mail. “We have to keep an open mind and allow for something yet unaccounted for in the data analysis. However, this possibility has been (and continues to be) under tight scrutiny and, so far, the tension only got stronger with further analysis.”
Nevertheless, many cosmologists wonder if the contradictions between theory and observation should be resolved by a modification of the theory of gravity. Possible improvements of general relativity have been studied by researchers for decades, but the most popular of them is a set of theories proposed in 1974 by Gregory Horndeski of the University of Waterloo, Canada, in which the details of how matter curves spacetime may differ from Einstein’s theory... (MORE - missing details)