Questions tagged [cosmology]

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

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4
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1answer
140 views

Critical density of the universe (no, not that one)?

I seem to recall reading something that said if the density of the universe was off by as little as one atom per cubic meter, the universe, as we know it, would not exist; one less and there wouldn't ...
2
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0answers
50 views

How the observed deuterium abundance constrains big bang nucleosynthesis?

I know deuterium can be used as a good tracer to constrain the big bang nucleosynthesis. But can anyone tell me the in details how this is actually done?
2
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0answers
130 views

How to to compute the expected number of collisions between stars when Milkyway and Andromeda merged? [duplicate]

Can some one tell me how you can roughly estimate the number of collisions when Milkyway and Andromeda merged with each other. I know it is somehow related to the mean free path of the star. But ...
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1answer
133 views

How does inflation justify the nonexistence of magnetic monopoles?

It is said that inflation justifies that magnetic monopoles don't exist. Can anyone explain how inflation theory explains the non existence of magnetic monopoles?
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1answer
236 views

Mass resolution

Can someone define mass resolution in terms of cosmological simulations such as the millennium simulation? Is it just the smallest mass that a dark matter particle can have and still be detected by ...
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2answers
245 views

Is the universe eternal?

I understand that as the universe continues to expand, eventually our supply of hydrogen will be exhausted and then it's "lights out." Is that really going to happen or is there something "eternal" ...
3
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1answer
157 views

Where are heavier elements than iron produced?

Specifically in the milky way, if that changes anything. I feel like it should be in the disk, since stars there are richer in heavier elements, but I'm not sure.
4
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1answer
61 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
78 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?...
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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
178 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
53 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) ...
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0answers
99 views

Why can primordial tensor perturbations of the CMB be ascribed to gravitational waves?

Why can primordial tensor perturbations of the CMB be ascribed to gravitational waves? Is this attribution unique, or are there other mechanisms that could lead to the excitation of tensor modes? In ...
0
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1answer
174 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
127 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 ...
1
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1answer
97 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 ...
3
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0answers
89 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?
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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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1answer
259 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
101 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^+}+...
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1answer
170 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
610 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
153 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
105 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://...
0
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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 ...
4
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1answer
554 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
96 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 ...
9
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2answers
179 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
233 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 ...
7
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2answers
808 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 ...
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1answer
5k 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 ...
8
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1answer
607 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, ...
22
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1answer
390 views

Dark Matter Particle Candidates

Dark matter appears to dominate the matter component of the universe as compared to luminous, or baryonic, matter. Though it does not interact electromagnetically (it doesn't absorb, scatter, or emit ...
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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
111 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 ...
2
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1answer
137 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 ...
5
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1answer
206 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 ...
3
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0answers
141 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 ...
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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 ...
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 ...
3
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1answer
125 views

What are the concrete technical arguments supporting the idea that the wave function of the universe can be written as partition function?

In this talk, the not yet settled down idea that the wave function of the universe could potentially be written as the partition function of a scale invariant statistical field theory is mentioned: $...
19
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3answers
2k views

Existence of gravitons?

For a lot of my uninformed life, I have doubted the existence of gravitons or even that gravity is an actual "force" (like electromagnetism). This is because my vision of general relativity was that ...
2
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1answer
510 views

Will Hubble's law always be accurate?

We have Hubble's law: $$v=Hd$$ where H is the Hubble parameter, which is decreasing in value, but it will be constant in the distant future. So, assuming the Standard Model of Cosmology holds true, ...
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1answer
112 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 ...
4
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1answer
272 views

Are galaxies moving away from us faster than before?

Are all galaxies moving away from us at constant speed even those that may be moving in our direction as the space is being formed? How does nothingness appear to push matter? A black hole sucks as ...
46
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2answers
2k views

Two species of dark matter?

At this point in time, evidence for the existence of dark matter has accumulated in many ways: it affects galactic rotation curves plays a major role in cosmology, and the evolution of structure in ...
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1answer
1k views

Three-torus model of the universe

I want to ask a question about the Three-torus model of the universe, as described in wikipedia: https://en.wikipedia.org/wiki/Three-torus_model_of_the_universe "is a proposed model describing the ...

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