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Questions tagged [observable-universe]

The observable universe of a given observer encompasses the volume of space from which information - particles, radiation - could ever (past, present or future) reach that observer.

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Is every object in the universe really expanding? Even small scale objects like humans? [duplicate]

So I was asked a question by someone that "if everything in the universe is expanding, then the lens we use to look at stellar objects is expanding as well, meaning we shouldn't be able to tell the ...
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Time in our Universe versus time in Black Holes

The universe is immense and 13.8 Billion years has passed overall since it was formed in the Big Bang event. If time greatly slows down inside a Black Hole (BH), then logically very much less than 13....
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How do they know the numbers of the energy pie chart of the universe?

They say that the total energy of the universe consists of 4.9\% ordinary matter, 26.8\% dark matter and 68.3\% dark energy. I hear this pie part in virtually every lecture in cosmology. How do we ...
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If the universe stopped expanding, would there be an infinite amount of reachable negentropy?

Given that the Universe's expansion is accelerating, stars are constantly leaving the reachable Universe. However, if the Universe wasn't expanding (nor contracting), we could forever reach further ...
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Existence of things outside the observable universe

I've read multiple physicists talk about stuff (e.g. galaxies, black holes, etc.) "existing" outside the observable universe, but it's unclear to me in what sense these things "exist". By definition ...
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When did the radiation domination end and matter domination start?

After the inflationary era the Universe became radiation dominated. The era of radiation domination is defined as the phase during which the temperature of the Universe was so high that the kinetic ...
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Can I make the visible universe charged by firing short lived charged quantum black holes? [closed]

Imagine I fires a couple of negatively charged quantum black hole and each black hole would last probably a second or two before releasing enormous amount of pure energy, haven't I single handedly ...
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Do we really need a new physics? [closed]

I have heard many times in news that a new discovery has taken place, and we need a new physics. It has recently been announced in scientific news that the world is apparently expanding faster. And ...
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54 views

Olber's paradox with Rational Numbers [closed]

Does Olber's paradox claim that in an infinite universe (both in space and in time) light should be able to reach you from any direction? Consider the thought experiment of placing an infinite number ...
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If the universe is expanding, then the distance between the atoms must increase, which should result in decrease in density of everything [duplicate]

In the expansion of universe theory, it is already written that the scale of universe is itself expanding, which means the consequence for expansion of universe is not that atoms are growing in size, ...
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35 views

Does the entire Universe prevent transforming the observable Universe into a black hole?

Consider the for us observable Universe (the entire Universe is many orders larger). It is (I think) common knowledge that the Schwarzschild Radius (SR) exceeds the radius of our observable Universe (...
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What is the co-moving distance to a point, at which the late-time acceleration of the universe began?

First of all sorry for my English - it is not my native language. According to the following wonderful diagram made by Pulsar, the late-time acceleration of the universe began at $t_{acc}=$ 7.7 ...
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Do Boltzmann Mechanics imply we're the only example of sentience in our observable universe? [closed]

My understanding of BM is that the Big Bang was just a statistical event. Under this model, many Big Bangs occur which result in a 2-power distribution of universes; there are twice as many universes ...
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Is there a cosmological model where the observable universe is bidirectionally infinite and yet finitely bi-limited?

Is there a cosmological model where the observable universe continues to shrink as we go back in time, and continues to expand as we go forwards in time, and such that at both directions it never ...
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Observable universe radius for distant observers

The radius of the observable universe is about 46 Gly. Is that figure true for all current observers in our universe? Is it true if the universe is finite or infinite, flat or curved?
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Is the Universe an isolated system? [duplicate]

I am not a physicist but this question has bugged me for a while now. The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. However, if the ...
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58 views

What will happen with the cosmic microwave background radiation if the Big Rip theory is true?

There are three things I can imagine happening: It appears one light year further away every year, which would be a bit weird, because it would be behind the universal event horizon, but still ...
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Observable universe thought experiment

Imagine 3 objects: A, B, and C, positioned along a straight line in space: A....................................B...................................C A and C move at the speed of light towards B. A ...
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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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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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108 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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78 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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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
102 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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4answers
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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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61 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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53 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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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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67 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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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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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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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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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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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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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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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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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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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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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
106 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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527 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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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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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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64 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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237 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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229 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 ...
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236 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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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 ...