The study of the large-scale structure, history, and future of the universe. Cosmology is about asking and answering questions about the "big picture" - the extent, origin, and fate of everything we know.

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1answer
87 views

Is there an alternative to dark matter?

I've been reading about dark matter. I understood that its existence is inferred by the discrepancy between the gravity exerted by a galaxy (total mass) and the movement of the stars that compose it. ...
0
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1answer
34 views

Friedmann equations question

Friedmann equations for critical density is: $$\rho_c = \frac{3H^2}{8\pi G}$$ Is there any other way to write this equation? For example: $$\rho_c = \frac{3}{8\pi GH^2}$$ I saw the above form on ...
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1answer
41 views

Data/signal from a black hole to observe a singularity

I wonder if a situation is possible where, we measure some signal/property concerning a black hole. Supposing the measurement we make with some telescope, gets us the Fourier transform coefficients of ...
7
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2answers
246 views

If we could build a telescope to view the cosmic neutrino background, what would we see?

If we could build a neutrino telescope capable of viewing relic neutrinos that decoupled after the big bang, with a similar angular and spectral resolution that is possible now for the CMB, what would ...
4
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2answers
1k views

Value of the Hubble constant over time

There is something I don't understand about the Hubble "constant" H, as it seems to clump two concepts together that I can't quite unify in my head. On the one side, we have $$V = D H$$ which means ...
3
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3answers
92 views

The Universe as a four-dimensional sphere?

I was chatting with my 12yo cousin yesterday and we got to the Universe, its size and stuff like that. Then he came up with the idea (I'll rephrase it), that the Universe could basically be a 4d ...
10
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2answers
1k views

Is dark matter repulsive to dark matter? Why?

I think I saw in a video that if dark matter wasn't repulsive to dark matter, it would have formed dense massive objects or even black holes which we should have detected. So, could dark matter be ...
5
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2answers
275 views

Questions about the Jeans length

I have a couple of questions about the Jeans length. Suppose the universe has a homogeneous energy density, except there's a spherical region that is overdense. I understand that if the region is ...
8
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1answer
52 views

Is it possible to determine whether distant galaxies are gravitationally bound

In a previous question, one issue was related to the potential energy of cosmic structures. This raised in particular the question of whether these structures are gravitationally bound. If you ...
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2answers
46 views

Searching for Big Bang Neutrinos

How is it possible to discover neutrinos from the big bang? Were the neutrinos emitted just before the big bang? Or at the same moment? If they moved outward in all directions, basically unhindered ...
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1answer
47 views

Degrees of freedom in the early Universe

I am reading Dodelson's textbook on cosmology. On page 66 we find equation 3.26: $$\rho = \frac{\pi^2}{30}T^4\biggl[\sum_{i=\text{bosons}}g_i+\frac{7}{8}\sum_{i=\text{fermions}}g_i\biggr]\equiv ...
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2answers
56 views

What is the composition of the universe's population of neutrinos?

I believe earth-based detectors measure mainly solar neutrinos, which have energies on the MeV scale of nuclear physics, are directed from the sun, and have flavors determined by the sun's nuclear ...
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0answers
25 views

What is holographic dark energy? [closed]

What is holographic dark energy? Or what is the relation between holography and gravity (or dark energy)?
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0answers
34 views

Sharpness of starlight

I am interested in argument that the observed lack of blurriness in starlight refutes the "tired light" theories. I have not been able to find this argument expanded in detail. There are two ways ...
1
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1answer
46 views

Expanding universe space through matter or matter through space? [duplicate]

So the universe is expanding, rather space is expanding. By expanding we mean space is coming into existence at all points.  Is that an equal rate of expansion everywhere? Now the expansion does not ...
2
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1answer
30 views

Sign of Hubble slow-roll parameters

Given the Hubble slow-roll parameters $\epsilon=-\frac{\dot{H}}{H^{2}}$ and $\eta=\frac{\dot{\epsilon}}{H\epsilon}$, can they assume negative values? For inflation to occurr they are required to be ...
24
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3answers
4k views

How can interstellar space have a temperature of 2-3K?

Several different sources online state that the average temperature of interstellar space (or the universe in general) is around 2-3K. I learned that temperature is basically the wiggling of matter, ...
1
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1answer
87 views

Expanding metre sticks [duplicate]

Given the Universe is expanding. Therefore everything within the U is expanding, in all dimensions, subatomic to cosmic. Then all metre sticks are expanding. The question: How can we measure U ...
7
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2answers
68 views

What did recombination look like?

I recently remembered that someone worked out what the big bang sounded like and that got me thinking... About 377,000 years after the Big Bang, electrons became bound to nuclei to form neutral ...
2
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3answers
115 views

Expansion of the Universe regarding Time and lightspeed

According to our current understanding, the universe is expanding... it is often said that every galaxy is moving away from us (I'm assuming that isn't true for galaxies in our local group), and it's ...
2
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2answers
63 views

How do we observe the expansion of the universe?

This is my first question. So I was reading about expansion of the universe and from what I've seen, the only way that we know that universe is expanding is by measuring redshift. Also, here it says ...
9
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3answers
2k views

What has been proved about the big bang, and what has not?

Ok so the universe is in constant expansion, that has been proven, right? And that means that it was smaller in the past.. But what's the smallest size we can be sure the universe has ever had? I ...
2
votes
1answer
77 views

Thermal gravitational radiation and its detection

To my poor knowledge on the topic, the gravitational waves that are most likely to be detected by LIGO or other experiments do not have thermal spectrum. But I'm not certain. I know that Hawking's ...
4
votes
1answer
97 views

Pressure and Density Using a General Lagrangian

Given a lagrangian of a form: \begin{equation}\mathcal{L}=f(\phi,\partial_{\mu}\phi\partial^{\mu}\phi)\end{equation} where $f$ is a function, I need to derive pressure and density in a FLRW universe ...
4
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3answers
265 views

What is the opposite of the Planck length?

What "large size" unit of length could be considered at the opposite end of spectrum from Planck's length? Is there a table of smallest and largest value for various physical quantities that can be ...
3
votes
1answer
681 views

Does dark energy annihilate energy?

Antimatter and matter particles annihilate. But does dark energy annihilate energy? We consider energy to be photons, and such, correct? So when we say energy, we're actually talking about some ...
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2answers
46 views

Tachyonic field

i'm working on a paper about symmetron cosmology. symmetron is a scalar field that by its symmetry breaking can explain the dark energy. the action is: ans A , V are assumed to be: where M ...
11
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1answer
234 views

As the universe expands, the wavelengths of photons are stretched, and energy is lost. What about electrons?

Will electrons, and other particles, also loose energy as they travel through the cosmos? They have wavelengths. Do they get "stretched"? My guess is that the EM force, somehow, counteracts this ...
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2answers
51 views

Why did the big bang need to produce equal amounts of matter and antimatter?

I've tried to find an answer for this but couldn't find one. Whats the problem with matter being created with little or no antimatter
2
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1answer
141 views

Difference between horizon and flatness problems & how inflation solves flatness (w/out math)

Layman here so I'm hoping for an answer for my query that doesn't involve math. I'm reading about inflation and how it solves the flatness and horizon problems. I get that the horizon problem deals ...
3
votes
2answers
109 views

Quantum entanglement on cosmological scales

This may be a foolish question given my limited understanding of QM but here it is. As I understand quantum entanglement basically means that two particles evolve as a single "unit", i.e., are ...
0
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1answer
21 views

Universe in the magnetic spectrum

Can we scan the universe in magnetic spectrum and then separate the paramagnetic , diamagnetic and ferromagnetic objects and get a new picture of the cosmos.
3
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3answers
267 views

Dark age of universe when all fusion process ceases?

Some say we live in the golden age of the universe because there exits countless number of stars that shines in the dark universe. As the supply of gas for star formation is steadily being exhausted, ...
2
votes
2answers
90 views

Expanding universe and the speed of light [duplicate]

When physicists talk about the expanding universe they often say that the distant galaxies are not really "moving" away but instead the space itself between us and them is expanding. If this is true ...
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2answers
112 views

Why is everything colder nowadays? [duplicate]

I know that when the universe began it was incredibly hot. Ever since, it is been cooling, and nowadays the average temperature of the universe is quite close to 0 K. Is this a consequence of having ...
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2answers
66 views

Best Research Documentation Habit for Computational Physics Research [closed]

I am currently doing research in a subset of the field of gravitational lensing called weak lensing. I am simulating blended galaxy profiles and creating an algorithm to extract the true parameters I ...
10
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2answers
151 views

Is the Big Bang defined as before or after Inflation?

Is the Big Bang defined as before or after Inflation? Seems like a simple enough question to answer right? And if just yesterday I were to encounter this, I'd have given a definite answer. But I've ...
3
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2answers
88 views

Are the implications of an infinite universe necessarily so unsettling

I have often heard it said (by professional cosmologists) that if the universe is infinite, then there necessarily exist infinitely many copies of me repeated throughout. The reasoning seems to be ...
1
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1answer
53 views

geodesic conjugate points

I was reading "Nature of space and time" by Penrose and Hawking, pg.13, If $\rho=\rho_0$ at $\nu=\nu_0$, then the RNP equation $\frac{d\rho}{d\nu} = \rho^2 + \sigma^{ij}\sigma_{ij} + ...
0
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0answers
20 views

How did the gravity of the particles/objects behave at the edge of the expanding universe moments after big bang?

During a very short time after the big bang, the universe must have had an edge of space-time which is very close to all the matter in the universe. The particles which are close to or on the edge ...
2
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1answer
61 views

What was a second in the early universe?

I have read some popsci articles and documentaries about the early universe and they often explain how various features of the universe came about and at what time. For example hydrogen atoms came ...
0
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2answers
817 views

How did the inflaton field “add” energy to the universe?

How did inflation add energy to the universe? What mechanism did this occur by? In other words, where did that energy come from? Was it due to the quantum fluctuation (or that scalar field rolling ...
0
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1answer
29 views

Negative Energy in Inflation Theory (Low/Zero Energy Universe)

I've been reading Max Tegmark's book: Our Mathematical Universe. It's very interesting, but I wanted to know more about one particular thing. The book simplifies things and I know inflation theories ...
0
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0answers
30 views

Does the zero energy universe hold true on some mathematical grounds besides observations?

I was reading Stephen Hawking's 'The theory of everything' when I came across a very interesting type of universe, the 'zero energy universe' since then, I've read some websites but all they used to ...
5
votes
4answers
62 views

Why doesn't the light from galaxies appear stretched? [duplicate]

Maybe it's my ignorance of astrophysics/cosmology, but I have been wondering this: Why do galaxies not appear stretched when we observe them? Assuming a galaxy that we observe is 100,000 light years ...
1
vote
1answer
28 views

Luminosity distance from angular diameter distance

Consider that I know the cosmological angular diameter distance at a given redshift : $$D_{A}\left(z\right)=\frac{x_{object}}{\theta_{observer}}$$ Is there a general formula to compute the ...
3
votes
3answers
86 views

Does or should the metric expansion of space imply a locally observable increase in kinetic energy?

The title is the question. Here's why it seems like local kinetic energy should increase: Numerous questions and answers here and elsewhere suggest that the reason the metric expansion of space is ...
3
votes
2answers
109 views

How does the gravity well change as space expands? [duplicate]

How does the gravity well change as space expands? If we assume that the Earth's gravitational field curves flat space to create a gravity well then how does the gravity well change as space expands ...
2
votes
2answers
128 views

Is time nothing but the speed of light (or the light itself)?

With regard to relativistic effects on time, all the examples and explanations revolve around light and its speed. Especially in explanatory situations that explain this using photon clock, it seems ...
12
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3answers
786 views

Is “now” or “the present moment” properly defined in GR?

My question is about the extent to which "now" is defined in GR. In Minkowski spacetime, it's possible to define a "now" for an inertial observer by finding a spacelike 3-plane such that, in the ...