3 ms·

So it seems like this may have a link to dark matter but not energy. I have no expertise in physics but it seems like dark is a bad term as it only reflects our

So it seems like this may have a link to dark matter but not energy. I have no expertise in physics but it seems like dark is a bad term as it only reflects our ignorance and inability to detect through normal electromagnetic means.

thekingofh 6y ago
Dark is just a placeholder until we find out exactly what's causing it.
at_a_remove 6y ago
Astronomers work through photons obtained and scientific inference on those photons. Lots and lots of inference. "Light" matter simply means "we can see it through a telescope because it gives off light," which means stars at a great distance, or closer up, light from a star reflected off of a surface. Dark, therefore, means "we know it is there but it isn't directly observable by gathering photons with a telescope."
- raattgift 6y ago
  Crucially we can show it's there by examining how it affects the light from bright background sources passing through it: there is a spectral shift without the imposition of new emission or absorption lines, and there is Einstein lensing [ https://en.wikipedia.org/wiki/Gravitational_lens https://en.wikipedia.org/wiki/Gravitational_lens ]. We can see this in the large by observing the http://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html http://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html on lines of sight through and near galaxy clusters. For example, [presentation abstract] https://depts.washington.edu/astron/event/tba-liang-dai/ https://depts.washington.edu/astron/event/tba-liang-dai/ [related paper https://arxiv.org/abs/1804.03149v2 https://arxiv.org/abs/1804.03149v2 ] or https://arxiv.org/abs/1706.07870 https://arxiv.org/abs/1706.07870 The tl;dr is that distant light sources (distance obtained by among other things comparing the angular diameter and brightness on the sky of spiral galaxies containing common bright sources within them such as supernovae or molecular clouds lit by quasars) are redshifted in a way that is very hard to explain without an accelerating expansion of space (which in the equations of the Standard Cosmology we represent as dark energy), and are lensed by luminous galaxy clusters in such a way that is very hard to explain without mass that doesn't emit or absorb light. "Hard to explain" because (just as one example) the Lyman-alpha line and its relativistic behaviour is extremely well studied in controlled laboratory conditions in various places in our solar system (notably sensitive experiments carried in various spacecraft probing other bodies in our solar system). In a number of spacecraft, Lyman-alpha photometry testing is used to determine the Hydrogen-Deuterium ratio of planetary atmospheres, since cells of those two gasses form practically ideal narrow-band rejection filters, and over time a mixture of cells reveals the atomic ratio of those isotopes in the atmosphere. People who observe Mars have used this technique to study the rate at which Mars is outgassing water vapour, in particular. As it is useful to planetary scientists to look through varying depths of a given planetary atmosphere backlit by distant luminous objects, in practice the cosmological Lyman-alpha forest can be sampled by orbiting Ly-a photometers "for free". You can't really reject the resulting Ly-a data without also rejecting the Deuterium/Hydrogen ratio obtained from probes on the surface of Mars, which happen to match. And the data typically also encodes other spectral lines principally from the intergalactic medium, and those lines are similarly redshifted and amplified by relativistic -- including general relativistic -- effects. Both dark matter and dark energy induce matching general relativistic effects, and nobody has figured out how to produce the same spectral features without including dark matter and dark energy.

thekingofh 6y ago

Dark is just a placeholder until we find out exactly what's causing it.

at_a_remove 6y ago

Astronomers work through photons obtained and scientific inference on those photons. Lots and lots of inference. "Light" matter simply means "we can see it through a telescope because it gives off light," which means stars at a great distance, or closer up, light from a star reflected off of a surface. Dark, therefore, means "we know it is there but it isn't directly observable by gathering photons with a telescope."

raattgift 6y ago
Crucially we can show it's there by examining how it affects the light from bright background sources passing through it: there is a spectral shift without the imposition of new emission or absorption lines, and there is Einstein lensing [ https://en.wikipedia.org/wiki/Gravitational_lens https://en.wikipedia.org/wiki/Gravitational_lens ]. We can see this in the large by observing the http://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html http://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html on lines of sight through and near galaxy clusters. For example, [presentation abstract] https://depts.washington.edu/astron/event/tba-liang-dai/ https://depts.washington.edu/astron/event/tba-liang-dai/ [related paper https://arxiv.org/abs/1804.03149v2 https://arxiv.org/abs/1804.03149v2 ] or https://arxiv.org/abs/1706.07870 https://arxiv.org/abs/1706.07870 The tl;dr is that distant light sources (distance obtained by among other things comparing the angular diameter and brightness on the sky of spiral galaxies containing common bright sources within them such as supernovae or molecular clouds lit by quasars) are redshifted in a way that is very hard to explain without an accelerating expansion of space (which in the equations of the Standard Cosmology we represent as dark energy), and are lensed by luminous galaxy clusters in such a way that is very hard to explain without mass that doesn't emit or absorb light. "Hard to explain" because (just as one example) the Lyman-alpha line and its relativistic behaviour is extremely well studied in controlled laboratory conditions in various places in our solar system (notably sensitive experiments carried in various spacecraft probing other bodies in our solar system). In a number of spacecraft, Lyman-alpha photometry testing is used to determine the Hydrogen-Deuterium ratio of planetary atmospheres, since cells of those two gasses form practically ideal narrow-band rejection filters, and over time a mixture of cells reveals the atomic ratio of those isotopes in the atmosphere. People who observe Mars have used this technique to study the rate at which Mars is outgassing water vapour, in particular. As it is useful to planetary scientists to look through varying depths of a given planetary atmosphere backlit by distant luminous objects, in practice the cosmological Lyman-alpha forest can be sampled by orbiting Ly-a photometers "for free". You can't really reject the resulting Ly-a data without also rejecting the Deuterium/Hydrogen ratio obtained from probes on the surface of Mars, which happen to match. And the data typically also encodes other spectral lines principally from the intergalactic medium, and those lines are similarly redshifted and amplified by relativistic -- including general relativistic -- effects. Both dark matter and dark energy induce matching general relativistic effects, and nobody has figured out how to produce the same spectral features without including dark matter and dark energy.