# 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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### Why the outer rim of our observable Universe is the oldest in age?

I don't get it. They telling us that the Webb space telescope will look far away from our home position deep into our observable Universe towards its outer rim about 13 Billion lyrs away where the ...
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### Can the age of the universe be much bigger than 13.8 billion

If observable universe is only a small fraction of the existing universe, does it imply that the age of the universe is much more than 13.8 billion years or the expansion of the universe is much ...
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### Large Scale Structure: Math proof that power spectrum being zero at a scale means fluctuations have variance of underlying Gaussian field

(Please don't close this one as a homework question. I am self taught and I have no way to get this information from any other source besides asking it here.) I am trying to solve this question (11.6) ...
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### The location of the exact center of the observable universe

If simultaneously in every direction, I were to precisely measure the distance to the edge of the observable universe (not: the physical universe), then would I find myself exactly in the center with ...
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### How is the expansion of the universe measured if redshift depends on the expansion itself? [duplicate]

To me this seems like a bit of a chicken egg problem. Based on the redshift of light (plus the assumption that physics worked the same way back then and there as it does now and here), we can ...
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### Is the inflation rate of the universe uniform throughout?

Is inflation constant at any given moment throughout the entire observable universe? I realize inflation was once much more prevalent, so at the edge of the observable universe, we would observe a ...
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### Distribution of matter in the Universe

My teacher explained today that quantum fluctuations in the early Universe (in particular, during inflation) determined the spatial distribution of small mass inhomogeneities, which, in turn, due to ...
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### Is it possible in principle to observe all galaxies in the observable universe?

Are there fundamental physical limitations that prevent the observation and cataloging of all (or almost all) galaxies in the observable universe? If there are no physical limitations, then what ...
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### What are the conditions needed for baryogenesis? [duplicate]

Physicists have created antimatter in the laboratory. But when they do, they create an equal amount of matter. That suggests that the Big Bang must have created matter and antimatter in equal ...
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### Why do some diagrams of the particle horizon, observable universe etc show the past light cone as hitting ~20glyr out from us at time zero?

This question is mainly in reference to this question: Is the observable universe equivalent to 'our' light cone?, and the answer, which is great. But what I can't wrap my head around is why ...
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### How fast is the edge of the observable universe expanding from the Earth?

If the universe is expanding at 73 kilometers per second per megaparsec, what is the fastest expansion speed that we can observe from Earth? I'm assuming that's the edge of the observable universe ...
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### What would the size of the observable universe be if you traveled 3/4 the speed of light?

I have a conceptual mess in my head and would like to clean it up. From the perspective of Earth, we can measure the observable universe diameter at current at 93BLY. I understand this and why/how ...
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### Opposite of event horizon - causality horizon or sphere?

Is there an opposite of event horizon - let's call it causality horizon or causality sphere - past which current objects in space can never be influenced from Earth because of the accelerating ...
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### Is "speed relative to the universe" a well-defined concept? [duplicate]

Prompted by commenting on this question. I offered the standard "Which frame of reference are you using? Yours? A satellite's? The sun's? The Milky Way's?" observation. Which prompted me to ...
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### Is it possible that the center of the universe is outside our observable universe?

Is it possible that the universe does have a center after all, but we just cannot see it because it already fell beyond the event horizon of our observable universe? If not, how do we know this for ...
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### Can a cluster of galaxies be partially inside Hubble's sphere and partially out of the cosmological event horizon?

Can a cluster of galaxies be partially inside Hubble's sphere and partially out of the cosmological event horizon? Let say we see only that part that moves in one orbital direction e.g. moving from ...
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### Why are the radius of the observable universe and radius of curvature of dark energy almost the same?

If you take the Einstein equation, $R_{\mu\nu} - \frac12 Rg_{\mu\nu} = \frac{8\pi G}{c^4}T_{\mu\nu}$ and plug in the estimated vacuum energy of $10^{-9} J/m^3$ for $T_{\mu\mu}$, you get a spatial ...
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### Another slices of the universe than space? [closed]

We observe a particular kind of "slices" of our universe: the slice that is called "space" (in the special sense of our 3 dimensional physical space). Are other kinds of slices in ...
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### What is the Poincare recurrence of the OBSERVABLE universe?

I have heard about a number called the Poincare recurrence time on Numberphile. It did not seem that exact in the way that it calculated this number, which was a power tower of 10s. What is the ...
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### Histories of the Universe under Different Initial Conditions

The history of the evolution of the Universe (we are talking about the observable part) on ultra-large scales (larger than the scale of galactic superclusters) under any initial conditions would be ...
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### How do we know the universe is still expanding?

We know the expansion of the universe has been occurring due to the red shift of light from neighbouring light sources. But given that light takes time to reach our telescopes, the glimpse of our view ...
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### Thinking on a 2D plane, Is the "point" of the big bang still active?

From what I understand, the light from CMB that we can observe is the result of the last scattering from the big bang which happened 380,000 years prior to the pattern we can see. Referencing: While ...
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### How do galaxies cross our particle horizon?

At the begin of his lecture "The Quantum Origin of the Universe" (2014) James Hartle makes the following statement: "A new galaxy with a 100 billion stars becomes visible - ie comes ...
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### Are unreachable parts of the universe 100% unreachable or do we just say so?

In Kurzgesagt's video TRUE limits of Humanity is it stated that most of the universe will be forever unreachable because the universe is expanding faster than the speed of light. This is possible ...
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### What is the average rate of passage of time in the observable universe relative to the passage of time on earth?

Main Question: If you were to average out the rate of the passage of time in the observable universe relative to earth, what would it be? Alternative Precise Question: What is the rate of passage of ...
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### Is the universe that we live in a Euclidean space? [closed]

I’ve been trying to wrap my head around general relativity, and it seems to me that what Einstein really did was give up the idea of a perfect Euclidean space. We idealize the fundamentals of physics ...
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### If the observable universe is only part of the universe, how do we know there is more matter than anti-matter?

I watched this youtube video about anti-matter, which says we don't know "why the big bang produced more matter than anti-matter". How do we know that more matter than anti-matter was ...
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### Could the observable universe be bigger than the universe?

First of all, I'm a layman to cosmology. So please excuse the possibly oversimplified picture I have in mind. I was wondering how we could know that the observable universe is only a fraction of the ...
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### How can the universe have an event horizon?

As I understand it (correct me if I'm wrong), the universe has an event horizon, and we can't possibly know if there's anything beyond it. This is due to the expansion of the universe, that space is ...
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### If we were able to prove that the universe is infinite, wouldn't that statistically prove that there is no other forms of life?

I want to begin my explanation using abstract mathematical explanation to repetition possibility by taking independent samples $X_n$ from some continuous probability distribution: https://math....
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### Matter moved faster than photons at big bang? [duplicate]

The observable universe is 90 billion light years across. But the age of universe is 13.8 billion years. Light would take 90 billion years to cross this distance, how could matter move that far in ...
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### How did we beat the light?

We observe that the universe is 13.8 billion years old, give or take a little. We can say this because we observe and measure the fact that distant galaxies and the CMB are that far away from us, ...
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### Can light reach far away galaxies in an expanding universe?

I've read that, since the universe is expanding at an increasing rate, light that leaves our galaxy now will never reach far away galaxies. That even though a galaxy is moving away from us slower than ...
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### Observable universe at $t=$infinity

On researching about observable universe I came across this excellent answer that explained Comoving vs Proper distance: https://physics.stackexchange.com/a/401181/281013 It answered why the edge of ...
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### What is the maximum amount of computation that can be performed in the future lifespan of the universe?

The ultimate question I have been trying to answer is the maximum universal population allowed by physical limits. Making some transhumanist assumptions I am perfectly happy with, I am equating this ...
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### Is the temperature of the universe rising?

I know the temperature of the universe is decreasing due to it's expansion after the big bang but after I came up with this article in AOP(please note I don't have the access of the journal,so I have ...
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### Does the Big Bang imply a finite quantity of matter in the universe, and that the universe is spatially finite (if boundaryless)?

If all matter was created in the Big Bang (not counting spontaneous generation of particle-antiparticle pairs), wouldn't that imply that a finite (if very large) amount of matter exists in the ...
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### Could we detect gravitational waves emitted outside particle horizon?

As we can see from a normal spacetime diagram like below, there's an overlap of the area within the event horizon and the area outside the particle horizon. Could we detect gravitational waves from ...
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### Can we produce the CMB power spectrum from a given model of inflation, theoretically? If yes, how?

To the best of my knowledge, the CMB power spectrum is obtained from a statistical analysis of the observed temperature anisotropies of the CMB sky. Is there a way of getting the power spectrum ...
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### Suspecting that the universe is even more bigger

Is it plausible that a possible extraterestrial scientist from a galaxy 13 bilion light years from us sees this part of the universe nothing else than just background cosmic radiation... and even more ...
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### Why isn't the 'Crisis in Cosmology' or 'Hubble Tension' explained by the Accelerating Universe?

The value of the Hubble parameter determined locally from the Type Ia supernovae is found to be greater than that determined from the Cosmic Microwave Background (CMB) radiation. This is known as the '...
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### Can objects within our Hubble sphere be influenced by objects outside of our Hubble sphere?

I feel like this is a silly question but I am having trouble wrapping my head around it. Lets use Andromeda as an example, since it has its own observable universe (slightly shifted from ours but with ...
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### Will we ever stop receiving CMB photons on earth? [duplicate]

If there is a lamp $1$ meter away from me which is switched on at time $t_1$ and switched off at $t_2$, I'll start receiving the light at time $t_1+\frac{1}{c}$ and stop receiving it at \$t_2+\frac{1}{...
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### What is the highest-energy event "naturally" happening in the current state of the Universe?

I know the question sounds a bit broad, but I will specify it a bit more. I also don't think there is a right answer necessarily, I am just interested in the scales of different processes happening ...
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### The future limit of the observable universe [closed]

Apologies in advance for the size of the question. Apologies also for English, I read well, but I can not write well so I use a translator. I thank everyone who will answer me. Guys, I started reading ...
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### How can the recession velocity of the galaxies be constant but the universe expand and a accelerating rate?

This almost seem counter-intuitive to me as if a object is such as a Galaxy is moving away from me at a constant velocity and the space between us is accelerating so should the velocity. Any help is ...
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### At which point in time in the history of our universe, was the observable universe exactly as big as the entire universe?

At which point in time in the history of our universe was the observable universe exactly as big as the entire universe? Does the Hubble Deep field represent such a time? Does this question make sense ...
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### Formula to get scales of the two components (radial and perpendicular) of Baryonic Acoustic Oscillations (BAO)

In an astrophysics context about BAO (Baryonic Acoustic Oscillation), it is written on the following paper BAO paper : "BAO are much smaller in amplitude than the CMB acoustic peaks, and are ...