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Questions tagged [observable-universe]

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Questions regarding the popular “finite but unbounded” universe

Regarding the finite but unbounded universe, according to my understanding some people come up with this idea because of the limitation of tools to detect what's "outside" the cosmic horizon (or "...
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Charge of multiverse

The charge of the universe is said to be zero. The most general argument I hear about this is that: since there was no net charge before the universe there must not be any after. Hence the universe is ...
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2answers
200 views

How can the universe be dodecahedron-shaped?

Physicsworld references "Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background" by J.-P. Luminet et al., published in Nature (...
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Large objects in the universe sized by gravitational and electrical constants

In a famous talk (from 14:30 to 15:00) Nima Arkani-Hamed tells us that what "fixes the size of the earth" is the equilibrium between the gravitational pressure against the atomic pressure of the atoms ...
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2answers
74 views

What is the biggest structure in the Universe?

The CMB cold spot (if it exists) may imply a supervoid of approximately 1 billion light years across. If the diameter of the Observable Universe is 46 billion light years this is about 2% the ...
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1answer
75 views

Where was the farthest object 13 billion years ago?

Asumme that we see a shape of the gallaxy which reached to us in 13 billion LYs. I understood that the object right now maybe in different shape and is >40 billon LYs away. But, as the universe is ...
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102 views

Nature of peaks in the CMB power spectrum and the underlying physical processes giving rise to them

The power spectrum is the Fourier transform of the two-point correlation function of a noise $X(t)$ where $t$ is time. It gives us the frequency content of a noise or in other words, tells how much ...
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0answers
47 views

Why is the observable universe 90 billion light years in diametre? [duplicate]

The question is fairly simple, we know that the universe has been expanding for 13.8 billion years, so how did they determine the size of the universe that we can observe, and why can we not peek ...
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1answer
91 views

Fraction of the Universe that is unobservable

I was watching this talk by Leonard Susskind. Near the end of his talk, he mentions how, because of the accelerating expansion of the Universe, there is a horizon to the Universe beyond which we ...
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21 views

Coincidental relation between ratio of masses, lengths and time intervals in the universe

The ratio between the largest and smallest lengths in the universe is about 10^41 and it's the same with time intervals. But for masses the ratio is about (10^41)^2. Is this purely a coincident or is ...
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4answers
177 views

Does the unobservable universe exist in the present?

Is our observation about universe limited due to the incapacity of telescopes to look further or is it just because we look too further into the future, so the "unobservable" universe hasn't been ...
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1answer
55 views

Is all matter within the observable universe?

The observable universe is everything within the range of the distance light can travel since the beginning of the universe. Since relativity states that matter cannot travel faster than light in any ...
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1answer
32 views

The surface of the last scattering and pictures of deep field

Maybe this question (in fact I have a few of them) is naive but nevertheless, let me try to ask it: it is concerning the furthest regions of our universe. Is the Hubble telescope capable to ...
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48 views

How much degenerate matter is there, compared to other states of matter?

I presume plasma is #1, while superfluid, BEC, QGP, etc, are pretty rare in nature, so the question is "how do solid, liquid, gas, and degenerate matter compare, by mass, in the known universe?"
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1answer
56 views

Observable universe size

When the Extremely Large Telescope (ELT) goes live in 2024 will it increase the size of our observable universe?
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2answers
68 views

How certain do science believe the universe is expanding?

The continue and increased rate of expansion is backed up by red shift, but that can also be true for a slowing down universe. Can the acceleration of expansion be reducing, still causing red shift ...
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3answers
887 views

Understanding the CMB background as a reference frame

We say the Earth is in relative motion with respect to the cosmic microwave background (CMB), causing anisotropies in the CMB spectrum. I have four very simple questions about this. How is it ...
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2answers
196 views

How do we know that the dark matter is cold or non-relativistic? [duplicate]

According to the $\Lambda$CDM parametrization of the Standard Model of Big Bang cosmology, the universe contains a cosmological constant $\Lambda$ associated with $73\%$ dark energy, $23\%$cold dark ...
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1answer
33 views

In theory, can we count every galaxy in the observable universe one-by-one?

Suppose I have a very advanced telescope that can detect long wavelengths of light, so I can deal with extreme redshift. I am also a very quick counter, so I can get through all of the galaxies before ...
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61 views

How sensitive is QFT calculation to (in)finiteness of our universe

We do not yet know whether our universe is finite or infinite - actually we may never know because we only see observable parts of our universe. Typical QFT calculations are done assuming that our ...
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1answer
60 views

How much has the universe expanded?

Expansion speed of universe is 68 km/sec, which is 1/4400 times the speed of light. From the big bang 13.7 billion years ago, the universe would have expanded a few hundred million light years at ...
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77 views

Definition of speed of photon coming from /galaxy moving away faster then the speed of light?

This is not the duplicate, because my question is not answered there, my question is about the definition of the speed of the photon and the galaxy itself. I understand that only photons reach us ...
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1answer
839 views

The difference between comoving and proper distances in defining the observable universe

"The radius of the observable universe is estimated to be about 46.5 Gly." If I understand correctly, it means the most distant object that we can see right now is 46.5 Gly away (at present), but it ...
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1answer
51 views

How many filaments are there in the observable universe? [closed]

How many filaments are there in the observable universe? I couldn't find any list on Wikipedia.
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2answers
405 views

Why do we think that the dark sector is simpler?

Matter makes up roughly $5\%$ of the Universe while dark matter makes up roughly $25\%$. For matter and radiation, we have a complicated Standard model with many generations of fermions and various ...
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1answer
96 views

Revisiting the flatness problem of the FRW Universe

The flatness problem in a nutshell One of the Friedman equation is given by $$ H^2\equiv\Big(\frac{\dot{a}}{a}\Big)^2=\frac{8\pi G}{3}\sum\limits_{i}\rho_i-\frac{k}{a^2}.\tag{1} $$ In terms of the ...
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1answer
329 views

Difference between causal horizon and particle horizon

The particle horizon is defined as the maximum distance up to which light can travel between two given times $t_i$ (often taken to be zero with $a(t_i)=0$) and $t$. This is the farthest distance in ...
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2answers
90 views

Is Flatness Problem still a problem since the observable universe is flat but the whole universe can be curved on much larger scale?

When we say the universe is flat, we mean just the observable universe. So that doesn’t mean the shape of the whole universe is also flat (ie. it can be curved on much larger scale) Is Flatness ...
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2answers
142 views

What is outside the boundaries of our universe (Future horizon) [duplicate]

I m having difficulty understanding what is after the "Future Horizon" as defined per wikipedia https://en.wikipedia.org/wiki/List_of_cosmological_horizons#Future_horizon I read the similar "If the ...
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1answer
61 views

Was time dilated in the early universe?

If energy is responsible for most of our mass, and mass is responsible for the effects on spacetime, then in the very early universe when its mass energy density was enormous, one second in time to a ...
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2answers
218 views

How can an infinite universe expand?

Physicists have estimated the universe to be flat and infinite, but I can't seem to understand what this actually means. First of all, what do they mean when they say that the universe is infinite, ...
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3answers
213 views

Why does the early universe look so big in a telescope? [duplicate]

As we look into the Deep Field, we see the Hubble photos of early galaxies up to only $400$ millions years old: Wikipedia: List of the most distant astronomical objects Because there is no center of ...
3
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1answer
182 views

Can forces from outside the observable universe be observed through a distant object? [duplicate]

I recently saw a video on dark flow from PBS space time on youtube, and it said that the flow had to be a constant motion, and not accelerating, because the object likely that caused the flow using ...
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1answer
67 views

Past size of the present observable universe? [closed]

I always had kind of a difficult time reconciling the idea that the Universe may be infinite beyond what we can ever observe, with the fact that the confines of the observable universe go all the way ...
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1answer
60 views

What happens to a terrestrial body bound to a far away galaxy?

Suppose that I could find an imaginary rope long enough to bind myself to a very distant planet, i.e. a planet within a very far away galaxy so that it is moving with the Hubble flow. To keep things ...
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0answers
85 views

Why is the radius of the universe larger than? [duplicate]

It's always been in the back of my head that the Universe is 13.8~ billion years old and that the Observable Universe is 46~ billion light years in radius. How is this so? It would logically be only ...
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1answer
602 views

Total positive energy in the (observable) universe

As long as I tried to find out what the total positive amount of energy is in the universe, the only answer I got was 'the total energy in the universe zero', which I do appreciate, but that's not ...
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0answers
121 views

How did Hubble come to the conclusion that the Universe is expanding?

From Edwin Hubble's measurement, the only thing that I can conclude is that the galaxies move away from Earth with their speeds proportional to their distances from the Earth. But until and unless ...
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1answer
129 views

Why is the Universe anisotropic to a non-comoving observer?

Our cosmology course instructor said that At a given cosmic time $t=\tau$, the comoving observers define a spacelike hypersurface $\Sigma$ in which the universe looks homogenous and isotropic. ...
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2answers
140 views

How does one measure the curvature $k$ in FLRW metric?

How does one measure the curvature parameter $k$ in the FLRW metric? $$ds^2=-c^2dt^2+a^2(t)[\frac{dr^2}{1-kr^2}+r^2d\theta^2+r^2\sin^2\theta d\phi^2]$$ In particular, what is the convenient equation (...
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1answer
74 views

Can we know if the universe is flat?

There are several cosmological models and ideas that deal with our universe's flatness problem, and they usually lead to topics like the Lambda-CDM model, the cosmological constant, vacuum genesis, ...
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1answer
2k views

Why is CMB not considered as the edge of the universe?

The estimates computed by WMAP and Planck state that the Big Bang occured about 13.82 billion years ago and recombination happened 379,000 years after that, which is when the universe first became ...
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58 views

Can we see enough of the universe to have a valid opinion on whether it's expanding?

Let me preface by saying I know very little about cosmology but have been wanting to learn. I'm a mathematician specializing in hyperbolic $3$-manifolds, and am aware of some of the applications of ...
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3answers
156 views

Why do some physicists think the universe could be infinite?

The deeper we look into space, the more primitive the structures are. Since we can see really primitive galaxies like Quasars, it's reasonable to think that there are even more primitive structures ...
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2answers
112 views

Size of Visible Universe: Why now ~50Bly instead of 15? [duplicate]

I understand, I think, the argument that the position is based on. Said in a sentence, the 15Bly's is in the temporal dimension only therefore space drops out because the redshifting absorbs the ...
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2answers
789 views

Non-observable universe vs last scattering surface

From Wikipedia, Observable Universe page https://en.wikipedia.org/wiki/Observable_universe Some parts of the Universe are too far away for the light emitted since the Big Bang to have had enough ...
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0answers
28 views

When will the Particle Horizon reach it's greatest extent?

If the extent of our observable universe at a point in time is determined by the particle horizon, and the future horizon tells us the point where we will never be able to see anything beyond is there ...
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76 views

Particle horizon with dark or phantom energy : $-1 < w < -\frac{1}{3}$, and $w < -1$

Consider a flat universe with curvature parameter $k = 0$ (for simplicity), with a dark fluid only, of state parameter $-1 < w < -\frac{1}{3}$ (lets call this "dark energy"), OR $w < -1$ ("...
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1answer
346 views

What is the hypersurface that represents the particles horizon?

I need to clarify a detail about the particles horizon (also called the causality horizon, not to be confused with the event horizon). For simplicity, consider an euclidian static universe, with a ...
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4answers
595 views

Does cosmic activity outside our observable universe affect us?

I am not a scientist or physicist, but i was wondering if the cosmic activities or events which might be on a super massive scale happening outside our observable universe, also affect us? Like the ...