Jan 1, 2021 09:19 PM
(This post was last modified: Jan 1, 2021 09:21 PM by C C.)
220 years ago today: 1st asteroid discovered
https://earthsky.org/space/jan-1-1801-di...y-of-ceres
INTRO: On January 1, 1801, the Italian priest, mathematician and astronomer Giuseppe Piazzi discovered the first asteroid, now called Ceres. It orbits in the asteroid belt, between the orbits of Mars and Jupiter. These days, Ceres is no longer classified as an asteroid, however. In 2006, the International Astronomical Union decided Ceres was big enough to be designated a dwarf planet. Ceres became the first-ever dwarf planet to be orbited by a spacecraft, from 2015 to 2018, when NASA’s Dawn mission peered down at Ceres and unlocked some of its mysteries... (MORE)
New data supports modified gravity explanation for dark matter, much to researcher surprise
https://www.universetoday.com/149416/new...searchers/
EXCERPTS: Dark matter is an extremely good theory. It’s supported by a wealth of observational and computational data, which is why it’s part of the standard model of cosmology. But dark matter hasn’t been directly observed, so sometimes even strong supporters of dark matter are motivated to look at the alternatives.
The most popular alternative is known as Modified Newtonian Dynamics (MoND), also known as modified gravity. The evidence we have for dark matter assumes that our understanding of gravity is correct. Both Newtonian gravity and general relativity have been strongly confirmed by observations, so the dark matter assumption is perfectly reasonable. But MoND assumes that on a fundamental level our understanding of gravity is slightly wrong.
[...] if MoND is correct, there should be a correlation between the rotation curve of a galaxy and the distribution of other nearby galaxies. This is where this new study comes in. The team used the Spitzer Photometry and Accurate Rotation Curves (SPARC) database to study the rotation curves of 175 galaxies.... Surprisingly, shockingly even, their study found a clear effect. ... Overall this is really good evidence for a single study. What’s more, the team expected this study to disprove MoND, so they are just as surprised by the results.
Overall this is a fascinating study. It doesn’t disprove dark matter, since numerous studies support the effects of dark matter, but it does support an aspect of modified gravity. It’s an unexpected result, and it needs to be studied further. MoND has long been out of favor among astronomers, but this study shows we shouldn’t believe the legend of its fall quite yet... (MORE _ details)
Is Einstein’s Cosmological Constant The Same As Dark Energy?
https://medium.com/starts-with-a-bang/as...731985b879
EXCERPTS (Ethan Siegel): . . . While matter (both normal and dark) and radiation become less dense as the Universe expands owing to its increasing volume, dark energy, and also the field energy during inflation, is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant.
The thing is, the cosmological constant is unlike the types of energy we know of otherwise. When you have matter in the Universe, you have a fixed number of particles. As the Universe expands, the number of particles stays the same, so the density goes down over time. With radiation, not only are the number of particles fixed, but as the radiation travels through the expanding Universe, its wavelength stretches relative to an observer that will someday receive it: its density goes down, and each individual quantum also loses energy with time.
But for a cosmological constant, it’s a constant form of energy that’s intrinsic to space. [...] There’s no way to “know” what the value of the cosmological constant is; we’ve simply assumed that it would be zero. But it doesn’t have to be; it could take on any value at all: positive, negative, or zero. If we extrapolate back in time — to when the Universe was younger, hotter, denser, and smaller — the cosmological constant wouldn’t have been noticeable. It would have been swamped by the much larger effects of matter and radiation early on. Only after the Universe has expanded and cooled so that the matter and radiation density drops to a low enough value can the cosmological constant finally appear.
That is, if there’s a cosmological constant at all.
When we talk about dark energy, it might turn out to be a cosmological constant. Certainly, when we take all of the observations we have so far, it appears that dark energy is consistent with being a cosmological constant, as the way the expansion rate changes over time agrees, within the uncertainties, with what a cosmological constant would be responsible for. But there are uncertainties there, and dark energy could be:
We would like to know, of course, whether it is a constant or not. The way we’re going to make this determination, as is always the case in science, is with superior and subsequent observations. Large data sets are the key, as is sampling the Universe at a wide variety of distances, as it’s the way the light evolves as it travels through the expanding Universe that allows us to determine — in gory detail — how the expansion rate has changed over time [...] By the end of the 2020s, we’ll have an enormous and comprehensive ground-based survey of the Universe...
It’s extremely tempting — and I’ll confess, I sometimes do it myself — to simply conflate the two, and assume that dark energy is nothing more complex than a cosmological constant. [...] But that underscores exactly why it’s so vitally important to make these novel measurements. If we didn’t bother to measure the Universe in a careful, precise, intricate fashion, we’d never have discovered the need for Einstein’s relativity in the first place. We never would have discovered quantum physics, nor would we have conducted most of the Nobel-winning research that’s driven society forward over the 20th and 21st centuries. 10 years from now, we’ll have the data to know whether dark energy differs from a cosmological constant by as little as 1%.
The cosmological constant may be the same thing as dark energy, but it doesn’t need to be. Even if it is, we’d still like to understand why it behaves this particular way and not any other... (MORE - details)
https://earthsky.org/space/jan-1-1801-di...y-of-ceres
INTRO: On January 1, 1801, the Italian priest, mathematician and astronomer Giuseppe Piazzi discovered the first asteroid, now called Ceres. It orbits in the asteroid belt, between the orbits of Mars and Jupiter. These days, Ceres is no longer classified as an asteroid, however. In 2006, the International Astronomical Union decided Ceres was big enough to be designated a dwarf planet. Ceres became the first-ever dwarf planet to be orbited by a spacecraft, from 2015 to 2018, when NASA’s Dawn mission peered down at Ceres and unlocked some of its mysteries... (MORE)
New data supports modified gravity explanation for dark matter, much to researcher surprise
https://www.universetoday.com/149416/new...searchers/
EXCERPTS: Dark matter is an extremely good theory. It’s supported by a wealth of observational and computational data, which is why it’s part of the standard model of cosmology. But dark matter hasn’t been directly observed, so sometimes even strong supporters of dark matter are motivated to look at the alternatives.
The most popular alternative is known as Modified Newtonian Dynamics (MoND), also known as modified gravity. The evidence we have for dark matter assumes that our understanding of gravity is correct. Both Newtonian gravity and general relativity have been strongly confirmed by observations, so the dark matter assumption is perfectly reasonable. But MoND assumes that on a fundamental level our understanding of gravity is slightly wrong.
[...] if MoND is correct, there should be a correlation between the rotation curve of a galaxy and the distribution of other nearby galaxies. This is where this new study comes in. The team used the Spitzer Photometry and Accurate Rotation Curves (SPARC) database to study the rotation curves of 175 galaxies.... Surprisingly, shockingly even, their study found a clear effect. ... Overall this is really good evidence for a single study. What’s more, the team expected this study to disprove MoND, so they are just as surprised by the results.
Overall this is a fascinating study. It doesn’t disprove dark matter, since numerous studies support the effects of dark matter, but it does support an aspect of modified gravity. It’s an unexpected result, and it needs to be studied further. MoND has long been out of favor among astronomers, but this study shows we shouldn’t believe the legend of its fall quite yet... (MORE _ details)
Is Einstein’s Cosmological Constant The Same As Dark Energy?
https://medium.com/starts-with-a-bang/as...731985b879
EXCERPTS (Ethan Siegel): . . . While matter (both normal and dark) and radiation become less dense as the Universe expands owing to its increasing volume, dark energy, and also the field energy during inflation, is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant.
The thing is, the cosmological constant is unlike the types of energy we know of otherwise. When you have matter in the Universe, you have a fixed number of particles. As the Universe expands, the number of particles stays the same, so the density goes down over time. With radiation, not only are the number of particles fixed, but as the radiation travels through the expanding Universe, its wavelength stretches relative to an observer that will someday receive it: its density goes down, and each individual quantum also loses energy with time.
But for a cosmological constant, it’s a constant form of energy that’s intrinsic to space. [...] There’s no way to “know” what the value of the cosmological constant is; we’ve simply assumed that it would be zero. But it doesn’t have to be; it could take on any value at all: positive, negative, or zero. If we extrapolate back in time — to when the Universe was younger, hotter, denser, and smaller — the cosmological constant wouldn’t have been noticeable. It would have been swamped by the much larger effects of matter and radiation early on. Only after the Universe has expanded and cooled so that the matter and radiation density drops to a low enough value can the cosmological constant finally appear.
That is, if there’s a cosmological constant at all.
When we talk about dark energy, it might turn out to be a cosmological constant. Certainly, when we take all of the observations we have so far, it appears that dark energy is consistent with being a cosmological constant, as the way the expansion rate changes over time agrees, within the uncertainties, with what a cosmological constant would be responsible for. But there are uncertainties there, and dark energy could be:
- increasing or decreasing in strength over time,
- changing in energy density, unlike a cosmological constant,
- or evolving in a novel, complicated fashion.
We would like to know, of course, whether it is a constant or not. The way we’re going to make this determination, as is always the case in science, is with superior and subsequent observations. Large data sets are the key, as is sampling the Universe at a wide variety of distances, as it’s the way the light evolves as it travels through the expanding Universe that allows us to determine — in gory detail — how the expansion rate has changed over time [...] By the end of the 2020s, we’ll have an enormous and comprehensive ground-based survey of the Universe...
It’s extremely tempting — and I’ll confess, I sometimes do it myself — to simply conflate the two, and assume that dark energy is nothing more complex than a cosmological constant. [...] But that underscores exactly why it’s so vitally important to make these novel measurements. If we didn’t bother to measure the Universe in a careful, precise, intricate fashion, we’d never have discovered the need for Einstein’s relativity in the first place. We never would have discovered quantum physics, nor would we have conducted most of the Nobel-winning research that’s driven society forward over the 20th and 21st centuries. 10 years from now, we’ll have the data to know whether dark energy differs from a cosmological constant by as little as 1%.
The cosmological constant may be the same thing as dark energy, but it doesn’t need to be. Even if it is, we’d still like to understand why it behaves this particular way and not any other... (MORE - details)