Questions tagged [cosmology]

Questions about the origin, history, evolution and fate of the Universe.

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5
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1answer
287 views

Collision with Andromeda Galaxy

According to Hubble's law, the universe is expanding and this expansion is proportional to distance between the two objects and thus objects are moving away at a very fast rate, then why is it ...
4
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1answer
62 views

Galactic revolutions relative to the age of the universe [duplicate]

If the universe is only 40 to 50 galactic revolutions old, how has it acquired its present state so "quickly" relative to this time scale?
3
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1answer
79 views

Does wavelength affect redshift caused by the metric expansion of space?

Does wavelength play a role in cosmological redshift? Do we see certain wavelengths affected less by expansion or notice any delay in arrival of certain wavelengths? How accurate are any observations?...
2
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2answers
193 views

How wary should I be of old cosmology books?

Recently I bought a couple of books on cosmology from a second-hand bookshop. One was published in 1993 and the other is from the 1970. I know, from a recent book, that the inflaton theory is ...
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1answer
188 views

Solving Friedmann equation for specific conditions

In Friedman equation: the case k < 0, with a Universe containing only matter (pressure = 0) so that $\rho = \rho_0/ a^3$ . What is the solution a(t) in a situation where the final term of ...
2
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1answer
55 views

galaxies in a specific radius in different shapes of universe

Let the distribution of galaxies be a number like $n$. we want to find the total number of galaxies within the radius $r$ from a specified point. in which shape of the universe (flat, open, closed) ...
3
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2answers
114 views

Question about $3p/c^2$ in Friedmann equation

The Friedmann Equation: $$\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}$$ I want to know, where does the $3p/c^2$ come from. I know it is something ...
0
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1answer
178 views

Wouldn't be “dark matter” better called “massless gravity”? [closed]

In spite of being a simple words game, that would make me feel less uncomfortable, as there could be a mysterious field still less mysterious than matter that passes through itself etc. Moreover, ...
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2answers
129 views

Why aren't other kinds of energies considered in the mass-energy of the universe?

Mass-Energy of the Universe: 5% ordinary matter, 27% dark matter, 68% dark energy What about other energies such as thermal energy, potential energy, kinetic energy, chemical energy, and radiant ...
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1answer
98 views

Universe and black holes

I know that the radius of the observable universe is about 46.6 billion light years and is expanding at every instant of time due to dark energy . So is it possible that we our entire universe could ...
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0answers
96 views

What is the difference between Steinhardt's and Neil Turok's cyclic cosmology and Guth's inflationary model and Hartle, Hawking no boundry cosmology?

I have been reading some cosmology and need to ask the experts at this point some specific points. I must admit I am a little confused. There are a few models of the universe and may be necessary at ...
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0answers
62 views

Does a 4-manifold universe imply zero net energy?

If our universe is a 4-manifold (i.e. with every point in space-time already existing) does this imply that there is zero net energy (as net energy would do work and so change the universe)? If so, ...
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1answer
103 views

Are there any physical phenomena that destroy information?

Are there physical phenomena, such as wave function collapse or black holes, which irrevocably destroy information about their prior states?
4
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1answer
1k views

How do I create a galaxy stellar mass function?

Suppose that I have an array of stellar masses of tens of thousands of galaxies, and that I also already know what the total "survey volume" is (in Mpc^3) within which all of those galaxies are ...
3
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1answer
1k views

Understanding The Turnover Point of Angular Diameter Distance

I am trying to get a better understanding of cosmological distances, in particular the angular diameter distance which I have also seen referred to as angular size distance. What I am looking for is ...
0
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2answers
292 views

Can gravity cause a Big Crunch, if the cosmological principle holds?

Say we have a closed universe that follows the cosmological principle. As a result, mass is distributed equally and, if as far as I know, gravity should have no net effect on the movements of galaxies ...
0
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1answer
406 views

Approximating density of hydrogen in [observable] universe

Let universe be completely made from hydrogen. And also we have redshift $z= 6$. with Hubble constant $H_{0} = 2.1941747572815535\times 10^{-18}\:\mathrm{s}^{-1}$. We also know that density of the ...
0
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1answer
262 views

Question about Hubble parameter (Hubble constant) and measuring it

I see this question in "An introduction to modern cosmology - Andrew Liddle - Wiley Publication": In the real Universe the expansion is not completely uniform. Rather, galaxies exhibit some ...
2
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1answer
107 views

Ionization Fraction of Helium in alternate, He dominated early universe

I'm asked to verify an expression for the ionization fraction of helium in a universe in which helium dominates baryonic matter. I'm given that the ionization fraction $X = \frac{n_{He^+}}{n_{He^+}+...
-1
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1answer
175 views

Could there be massive charged “rivers” in interstellar space?

Some suggest that there might be currents of charged particles moving at 0.98c in interstellar space. Since this is tainted by association with the crack “Electric Universe” cosmology, my first ...
1
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1answer
632 views

Finding equation for energy density of matter

I'm trying to get an equation for the energy density of matter of the universe $\rho(t)$, assuming the number of particles is conserved and rest mass energy is much greater than kinetic energy. $\rho(...
3
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1answer
155 views

Equating critical energy density to matter density

I'm asked first to calculate the critical energy density $\rho_{crit}$ of the universe given certain constants. The value I got was $5500\,\mathrm{MeV}\,\mathrm{m}^{-3}$. Then I'm asked to calculate ...
8
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1answer
106 views

Neutrino producton rate over time

Has the rate of neutrino production in the universe varied over time? Are there more or less being created now against the early days?
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0answers
47 views

DESDM COADD image access

I'm just starting a project to analyze some of the images obtained from the Dark Energy Survey (DES), and I'd like to get some of the COADD images from the SV data. For example, this paper: https://...
4
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1answer
586 views

How can the observable universe shrink in a Big Rip?

As far as I know, the Big Rip occurs when the scale factor reaches infinity in a finite time. This will only happen in a universe dominated by phantom energy (i.e. a universe with an equation of state ...
2
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1answer
97 views

Could the accelerating expansion of universe inidicate we are surrounded by “denser” space? [closed]

I'm quite novice about all this, only have read some vulgarization books and sites, but I wonder: could all the observable universe be falling outwards on some non-observable, more compact kind of ...
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2answers
181 views

How Good Are the Upper Limits on Heavy Elements?

There are between 90 and 254 stable nuclei all the way up to element number 82. In discussions and graphs about big bang nucleosynthesis nothing above lithium is even mentioned. It's a pretty safe bet ...
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1answer
159 views

Number of photons in universe? I'll take number from a star in unit time and extrapolate

I'll take number per average star, multiple by appropriate numbers , star/Galaxy, galaxies/universe, 14 billion years, and try to figure their location. I was looking for a rough number and a ...
31
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5answers
6k views

Is the universe considered to be flat?

I've read various articles and books (like this one) stating that we are not certain about the geometry of the universe, but there were experiments on-going or planned that would help us find out. ...
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1answer
238 views

can a life exist outside the universe? [closed]

can a life exist outside of universe. but we know that in our universe many galaxies have life without water and oxygen and it is possible that life exist outside the universe so help me to find the ...
3
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1answer
744 views

Latest cosmological parameters

I'm looking for the latest values (with uncertainties) of the four main cosmological density parameters $\Omega_i$ : \begin{align}\tag{1} \Omega_{\text{mat}} &={} ?, &\Omega_{\text{rad}} &=...
8
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1answer
649 views

What percentage of the hydrogen today has never been in a star

It stands to reason that some of the hydrogen and helium that formed directly as a product of the big bang might never have fallen into a star to re-ejected when that star explodes. My question is, ...
30
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1answer
6k views

Why didn't the Big Bang produce heavier elements?

Shortly after the Big Bang, temperatures cooled from the Planck temperature. Once temperatures lowered to 116 gigakelvins, nucleosynthesis took place and helium, lithium and trace amounts of other ...
2
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0answers
105 views

Galactic Filaments and their connection to Galaxy centre

If alliptical galaxies seem to a line with their associated galactic filament and that orientation is related to connections between the mass of said galaxy and the dominating central object, which ...
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1answer
113 views

How can space expand in accelerating speed when dark energy is constant?

I came to know that dark energy is constant.As mentioned in the Friedmann equations How can a constant energy cause an ...
7
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2answers
835 views

How can gravity lead to the Big Crunch scenario?

According to modern cosmology, space is expanding, causing proper distances (but not comoving distances) to increase between galaxies. In the Big Crunch hypothesis, gravity halts and reverses the ...
2
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1answer
142 views

Does the redshifting of photons from the Universe's expansion violate conservation of momentum?

The energy-momentum relation, $$E^2 = m^2c^4 +p^2c^2,$$ lets us derive the momentum of a massless particle: $$p = \frac{E}{c} = \frac{h\nu}{c}$$ However, the expansion of the Universe redshifts ...
3
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0answers
147 views

What is the spectral power output of a typical galaxy?

I'm looking for a function which gives the typical isotropic spectral power of light emitted by an entire galaxy on the whole electromagnetic spectrum (in watts per frequency unit), as a function of ...
21
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1answer
3k views

Does the recent news of “ten times more galaxies” imply that there is correspondingly less dark matter?

Nature: Universe has ten times more galaxies than researchers thought NASA feature: Hubble Reveals Observable Universe Contains 10 Times More Galaxies Than Previously Thought Headlines sometimes ...
5
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1answer
209 views

Why are galaxies distributed as walls and voids, or spider webs?

I have wondered why the "walls and voids" of the cosmological galaxy distribution seem to resemble a "foam", like rising bread that's ready to fall, with "voids" corresponding to empty bubbles in the ...
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0answers
66 views

Cosmology: about the equations of expansions

Suppose we assume that the gravitational constant is varying with time: $$G=G_0 e^{-\frac{t}{t_0}}$$ where $t$ is the time from the Big Bang, and $G_0, t_0$ being some constants. Assuming matter-only ...
-4
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1answer
119 views

Why Did The Universe Expand? [duplicate]

If all of the mass and energy in the universe was in a point smaller than the size of an electron at one time, why did it expand? Did it necessarily expand immediately on formation, or could it have ...
3
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2answers
357 views

What is left of a universe where no matter and energy exist and is there a term for this?

In the final energy state of the universe, there exists a hypothesis that all matter will have fallen into black holes and neutron stars. If, through Hawking radiation, after all matter is consumed ...
5
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3answers
475 views

How did the Milky Way and Andromeda Galaxy separate before they began their collision course?

How did the Milky Way and Andromeda Galaxy separate before they began their collision course? I understand that gravity is pulling together the two galaxies, but what separated them in the first ...
2
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1answer
460 views

Is there anything like a supermassive white hole? How are they formed? And where are they present?

The idea of a 'super massive white hole' is sometimes discussed regarding GR,SR and the destination of material 'lost' in a black hole. Is there any good theoretical or observational evidence for ...
2
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0answers
53 views

How many times has the sun orbited the galaxy? [duplicate]

Even if our solar system is not in a stable orbit around the center of the galaxy (is it?), since the birth of our sun, how far has it traveled relative to the center of the galaxy?
2
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1answer
169 views

How can we use hypervelocity stars to determine the origins of the Universe?

I was reading this article finding evidence of Universe's origin, which describes that in 1 trillion years we may lose the ability to determine how the universe was created. The answer seems to be ...
26
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7answers
5k views

The laws of physics through out the universe

How do we know laws of physics are the same throughout the universe? Intuitively I would say they would vary in two natural ways: the constants in the equations may vary or the math in the equations ...
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1answer
71 views

Can we observe the Milky Way on the edge of the universe? [closed]

If you traveled outward and outward in a straight line, indefinitely and pugnaciously, you would never arrive at an outer boundary. Instead, you would come back to where you began. Hence, in the same ...
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2answers
2k views

Which are the most energetic events in the Universe?

That's the question. I know about gamma ray bursts, is there anything more energetic?

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