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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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Does the observable part of universe become smaller, larger or the same near event horizon?

I am pondering a lot about the nature of event horizons, but this one is not obvious to me. When you are in a gravitiational well, does the boundary of observable universe become closer to you or ...
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Density fluctuations of the present Universe with Large scale structures

Disclaimer The question below is based on a vague knowledge, and hence, statements can be potentially wrong or misleading. An important quantity in Cosmology is the density fluctuation of matter ...
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What is the probability of a photon being emitted by the sun actually being absorbed by matter in the visible universe?

In the Hubble ultra-deep field image it is apparent that the visible universe has a lot between us and our light horizon, but what is the likelihood that a photon emitted by our sun actually "hits" ...
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Do states of two particles contain almost all information of their observable universe?

I am asking the question as a layman. If things are interacting with every other things in their observable universe, and if one knows the particular states, for example, the electromagnetic force, ...
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About space/timelike intervals and event horizon of observed universe

How does interval between us (e.g earth) and the spacetime outside of observable universe (above the event horizon) can be described in terms of timelike and spacelike intervals? Is it just spacelike ...
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If the observable universe's mass & energy was converted to white light, how many photons would there be?

Background: The idea is to get a large number that is relatable to humans. The number of atoms in the observable universe is estimated at around 10^80, but this number isn't satisfying as the ...
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Why is light, that was emitted right at the big bang (same time/place), arriving to us at different times [duplicate]

I understand the big bang to have (at least mathematically) started our universe at a virtually precise moment and within a virtually infinitesimal volume. Yet some of the light from that moment/...
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If A Very Distant Galaxy Has Never Been Observable By Us, Then It Never Will Be? [duplicate]

I understand that distant galaxies that are currently inside our particle horizon will, sometime in the future, not be observable by us anymore because they are receding faster than the particle ...
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A universal wave form and the observable universe

This may have been on the table before but I don't find such a question directly. If we started to approach the concept of a universal wave function, how would we consider the matter outside the ...
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About the applicability of the FRW metric in the present epoch vs the time of last scattering

The form of the metric of the Universe, the FRW metric, is obtained from the principles of spatial homogeneity and isotropy which are supported by observation. If we probe the present Universe, it ...
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Increasing the view of observable universe

I have read that the maximum observable universe is about 92 billion light years in diameter by measuring the microwave background waves emitted after some few million years ago from the big bang and ...
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Ambiguity in applying Newton's shell theorem in an infinite homogeneous universe

Newton's shell theorem has two corollaries: The gravitational attraction of a spherically symmetric body acts as if all its mass were concentrated at the center, and The gravitational acceleration ...
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How is observable universe so big if the universe is so young? [duplicate]

The diameter of observable universe is 93 billion light years but the age of universe is only estimated to be 14 billion years. So how does light have 46.5 billion years to travel from the boundary of ...
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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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1answer
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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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1answer
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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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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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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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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
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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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149 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
371 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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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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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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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 ...