Dark Energy, Dark Matter

How does science work? And what's all this about quantum mechanics?

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Astro Cat
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Dark Energy, Dark Matter

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I began a thread in The Lounge trying to get to know other members and I wanted that thread to stay on topic. So, I'm moving over some questions that appeared there to be discussed on their own. For background, I'm a researching astrophysics grad student (which is how these questions arose in the first place).
Age wrote: Fri Jul 01, 2022 4:35 am
Astro Cat wrote: Thu Jun 30, 2022 12:57 pm
Age wrote: Thu Jun 30, 2022 12:46 pm What is 'quiescence in galaxies', from your perspective? And,

What is 'dark energy', to you?
Quiescence is when a galaxy is quenched (no longer has a healthy star formation rate).
Which is said to be less than what, exactly?
The threshold for quiescence is somewhat arbitrary, but several authors place it at < 10^-11 stellar masses worth of stars /yr. For comparison, the Milky Way produces about 3 stellar masses of stars per year (and the Milky Way is on its way to quenching).
Age wrote:
Astro Cat wrote: Thu Jun 30, 2022 12:57 pm Quenching is associated with a lot of different aspects of galaxies such as their morphology (spheroid vs. disky), Sérsic index (drop-off in intensity as a function of distance from the center), AGNs (active galactic nuclei, e.g. their black holes), etc. We observe a buildup of high stellar mass spheroid galaxies with low specific star formation rates at lower redshifts (so, more recent in cosmic time) than at high redshifts, but it's tough to discern what common features of quiescent galaxies might have a causal role in their quenching or what features result from quenching.

Dark energy is any energy density with a negative equation of state that doesn't scale with the scale factor of the universe. It drives the acceleration of cosmic expansion
So, to you, the alleged acceleration of expansion of the Universe is caused by any energy density with a negative equation of state that does not scale with scale factor of the Universe, correct?

If this is correct, then what is the scale factor of the Universe, exactly?
The scale factor of the universe, a, is defined as 1 for the present universe and is used to relate the scale of the universe at different times (and is directly related to redshift, since looking out is looking back). For instance, a = 0.5 would be the time when the universe had half of its present size.

For instance, if we look out to redshift z = 1.1, we see the universe as it was when it was a little bit less than half of its present size:
a = 1/(1 + z) = 0.476

We can actually see the effect of this at the point of angular diameter turnaround. Funnily enough, the best way I've ever seen to describe that is with an xkcd comic (which I've absolutely included in slides I've used on the topic, LOL ^_^):

Image
Age wrote:
Astro Cat wrote: Thu Jun 30, 2022 12:57 pm and currently dominates the energy density of the universe, even over matter (baryonic and dark alike).
True?

So, what does the energy density with a positive equation of the state that does scale with the scale factor of the Universe actually do, exactly?
Energy densities with a positive equation of state (w = 1/3) and scales by /a^4 is simply radiation.

Energy densitites with an equation of state w = 0 and scales by /a^3 is matter.

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attofishpi
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Re: Dark Energy, Dark Matter

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May the force B with U.
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Astro Cat
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Re: Dark Energy, Dark Matter

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attofishpi wrote: Fri Jul 01, 2022 5:17 am May the force B with U.
Don't give me flashbacks. I had to slog through Jackson's "Classical Electrodynamics" (notorious for giving grad students aneurisms) for two semesters of grad E&M. Big shudder.

Book's still on my shelf. I still contemplate burning it.
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Re: Dark Energy, Dark Matter

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Astro Cat wrote: Fri Jul 01, 2022 5:12 am The threshold for quiescence is somewhat arbitrary, but several authors place it at < 10^-11 stellar masses worth of stars /yr. For comparison, the Milky Way produces about 3 stellar masses of stars per year (and the Milky Way is on its way to quenching).
Hello over here too, can a Milky-way sized or bigger galaxy with 2 or more supermassive black holes orbiting each other in the centre, become quiescent?
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Re: Dark Energy, Dark Matter

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..personally, I don't see the correlation.
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Re: Dark Energy, Dark Matter

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Atla wrote: Fri Jul 01, 2022 6:08 am
Astro Cat wrote: Fri Jul 01, 2022 5:12 am The threshold for quiescence is somewhat arbitrary, but several authors place it at < 10^-11 stellar masses worth of stars /yr. For comparison, the Milky Way produces about 3 stellar masses of stars per year (and the Milky Way is on its way to quenching).
Hello over here too, can a Milky-way sized or bigger galaxy with 2 or more supermassive black holes orbiting each other in the centre, become quiescent?
hi Atla!

Yes, in fact it's probably more likely than not. AGNs, or active galactic nuclei, are super correlated with quenching. We don't know whether quenching causes AGNs or if AGNs cause quenching though. That is part of my research.
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Re: Dark Energy, Dark Matter

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attofishpi wrote: Fri Jul 01, 2022 6:13 am ..personally, I don't see the correlation.
I thought you were making a joke (B is used for magnetic fields, U for energies)
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Re: Dark Energy, Dark Matter

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Astro Cat wrote: Fri Jul 01, 2022 7:11 am
Atla wrote: Fri Jul 01, 2022 6:08 am
Astro Cat wrote: Fri Jul 01, 2022 5:12 am The threshold for quiescence is somewhat arbitrary, but several authors place it at < 10^-11 stellar masses worth of stars /yr. For comparison, the Milky Way produces about 3 stellar masses of stars per year (and the Milky Way is on its way to quenching).
Hello over here too, can a Milky-way sized or bigger galaxy with 2 or more supermassive black holes orbiting each other in the centre, become quiescent?
hi Atla!

Yes, in fact it's probably more likely than not. AGNs, or active galactic nuclei, are super correlated with quenching. We don't know whether quenching causes AGNs or if AGNs cause quenching though. That is part of my research.
Interesting! I would have guessed that the AGNs orbiting each other would disturb the rest of the galaxy
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Re: Dark Energy, Dark Matter

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Astro Cat wrote: Fri Jul 01, 2022 7:12 am
attofishpi wrote: Fri Jul 01, 2022 6:13 am ..personally, I don't see the correlation.
I thought you were making a joke (B is used for magnetic fields, U for energies)
No, well not in any intelligent way clearly.

RE Correlation, I would have at least put in brackets (that everything within the galaxy has SOME correlation)

..but carry on, Bob.
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Re: Dark Energy, Dark Matter

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Atla wrote: Fri Jul 01, 2022 7:14 am
Interesting! I would have guessed that the AGNs orbiting each other would disturb the rest of the galaxy
To be more clear, even if it's two black holes, it would just be called "an AGN," and having two black holes doesn't make it an AGN. AGNs are when the black holes are actively consuming materials and making huge plasma jets.

It's just more likely that a binary system is going to disrupt things and become an AGN.
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Re: Dark Energy, Dark Matter

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Astro Cat wrote: Fri Jul 01, 2022 7:20 am
Atla wrote: Fri Jul 01, 2022 7:14 am
Interesting! I would have guessed that the AGNs orbiting each other would disturb the rest of the galaxy
To be more clear, even if it's two black holes, it would just be called "an AGN," and having two black holes doesn't make it an AGN. AGNs are when the black holes are actively consuming materials and making huge plasma jets.

It's just more likely that a binary system is going to disrupt things and become an AGN.
Right, so as I was wondering above, can a Milky-way sized or bigger galaxy with two or more supermassive black holes orbiting each other in the center, become quiescent? Or do they disturb the rest of the galaxy too much that would lead to increased star formation?
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Re: Dark Energy, Dark Matter

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I am unfamiliar with the diets of black holes, but what happens when they consume each other?

-Imp
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Re: Dark Energy, Dark Matter

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Astro Cat wrote: Fri Jul 01, 2022 5:12 am I began a thread in The Lounge trying to get to know other members and I wanted that thread to stay on topic. So, I'm moving over some questions that appeared there to be discussed on their own. For background, I'm a researching astrophysics grad student (which is how these questions arose in the first place).
Age wrote: Fri Jul 01, 2022 4:35 am
Astro Cat wrote: Thu Jun 30, 2022 12:57 pm

Quiescence is when a galaxy is quenched (no longer has a healthy star formation rate).
Which is said to be less than what, exactly?
The threshold for quiescence is somewhat arbitrary, but several authors place it at < 10^-11 stellar masses worth of stars /yr. For comparison, the Milky Way produces about 3 stellar masses of stars per year (and the Milky Way is on its way to quenching).
Every galaxy would be on their way to quenching, correct?

If no, then what is the opposite of quenching called?
Astro Cat wrote: Fri Jul 01, 2022 5:12 am
Age wrote:

So, to you, the alleged acceleration of expansion of the Universe is caused by any energy density with a negative equation of state that does not scale with scale factor of the Universe, correct?

If this is correct, then what is the scale factor of the Universe, exactly?
The scale factor of the universe, a, is defined as 1 for the present universe and is used to relate the scale of the universe at different times (and is directly related to redshift, since looking out is looking back). For instance, a = 0.5 would be the time when the universe had half of its present size.
When you use the word 'universe', what are you referring to, exactly?

And, if the scale factor of the universe is just defined as 1 for the present universe, then at every moment one is looking and observing from, half the time between that moment and when the so-called 'big bang' occurred would just be half the size correct?
Astro Cat wrote: Fri Jul 01, 2022 5:12 am For instance, if we look out to redshift z = 1.1, we see the universe as it was when it was a little bit less than half of its present size:
a = 1/(1 + z) = 0.476
This, what you call, 'looking out to redshift', I have already partly talked about and explained in this forum is just looking back, 'temporally'.

I have also partly explained how the looking at redshift then gets misinterpreted as the whole Universe is expanding, which effects the way the people with this misinterpretation then look at what is actually occuring. This misinterpretation was like when people would look at sun's movement and misinterpret this as "evidence" for a geocentric universe, which would then effect the way they look at, and thus see, what is actually occuring. That is; that it is not the Universe, Itself, that is expanding, but rather just a part of the Universe that is expanding.

How all of this happens and occurs can be shown, explained, or illustrated, for those that are interested.
Astro Cat wrote: Fri Jul 01, 2022 5:12 am We can actually see the effect of this at the point of angular diameter turnaround. Funnily enough, the best way I've ever seen to describe that is with an xkcd comic (which I've absolutely included in slides I've used on the topic, LOL ^_^):

Image



Energy densities with a positive equation of state (w = 1/3) and scales by /a^4 is simply radiation.

Energy densitites with an equation of state w = 0 and scales by /a^3 is matter.
You answered and clarified somewhat what they 'are', but not what they 'do', which was what I was asking for, and thus looking for to be clarified.
Astro Cat wrote: Fri Jul 01, 2022 5:12 am Image
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Re: Dark Energy, Dark Matter

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Atla wrote: Fri Jul 01, 2022 1:55 pm Right, so as I was wondering above, can a Milky-way sized or bigger galaxy with two or more supermassive black holes orbiting each other in the center, become quiescent? Or do they disturb the rest of the galaxy too much that would lead to increased star formation?
Yes, galaxies with binary supermassives can become quiescent, and it's probably more likely since stirring things up will turn them into an AGN.

One of the possible mechanisms of quenching is when an AGN starts firing plasma jets, it heats up gases too much to condense into star-forming densities.

Materials for star formation also come from filaments between galaxies (forming the cosmic "web" you might have seen), AGNs can be disruptive to infalling gas.
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Re: Dark Energy, Dark Matter

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Impenitent wrote: Fri Jul 01, 2022 4:29 pm I am unfamiliar with the diets of black holes, but what happens when they consume each other?

-Imp
Black holes will just merge into a single hole with a larger event horizon that must have a larger event horizon area than the mergers' combined thanks to thermodynamics (surface area is an equivalent to temperature; so making an event horizon that's exactly the same surface area as the merging holes' horizons would be the equivalent of a 100% efficient reaction)
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