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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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Negative Horizon distance

Consider a flat universe, here, proper distance can be given by R-W Metric: $$d_p (t_0) = c\int_{t_e}^{t_0}\frac{dt}{a(t)},$$ $t_e$ is the time when a photon is emitted from a distant galaxy, $t_0$ is ...
Polaris5744's user avatar
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Can the observable mass within the observable universe be estimated without using $G$?

The estimation of the baryonic mass of the observable universe can be done through its density parameter and the critical density, which depends on the gravitational constant $G$. Is there a way to ...
Manuel's user avatar
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Horizon problem, what if our observable universe is roughly equal to the whole universe, especially in early times?

How do we know that at a time of 380.000 years, when CMB got free, the observable universe was not equal to the actual universe? Maybe they were roughly the same and couldn't that explain the horizon ...
God's user avatar
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Does going to the other side of Milky Way galaxy mean seeing different observable universe?

What if there was a robot that got sent to the other side of the Milky Way by just traveling with a fusion engine and started orbiting a planet that has around Earth's gravity in order to avoid any ...
Roghan Arun's user avatar
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If the observable universe had only one galaxy, how would people know the expansion of the universe?

Hubble measured high redshifted galaxies to discover the cosmic expansion. In a hypothetical universe where only one galaxy exists, would there still be observational evidence for the Big Bang theory? ...
user74750's user avatar
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Can LISA observe BH beyond the observable universe?

Reading the paper "Astro2020 Science White Paper Where are the Intermediate Mass Black Holes?", and the plot in Fig.1, page 5, it seems LISA can see IMBH beyond 100 Gpc...Since the ...
riemannium's user avatar
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2 votes
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Will cosmic microwave background become invisible in the future?

If my understanding of CMB and Hubble's Law is correct, then CMB photons emitted from more than ~14.4 Glyr during Recombination Epoch would not reach us. The reason is this would correspond to Hubble'...
FritzS's user avatar
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Why can't we see past the observable universe?

Why can't we see light from beyond the observable universe? I've done a lot of research on this and all I've found is unsatisfactory answers and straight up nonsense. Some claim that the universe &...
Krokodil's user avatar
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What is the rough distribution for number of galaxies with a given mass in the observable universe?

Is there a rough formula for the fraction of galaxies in the observable universe with masses between M and M + dM? Or perhaps a graph that displays the same information? I've looked online but can't ...
Thanos's user avatar
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What was the size of the observable universe at the matter-dominated era and its mass density?

For instance, at the scale factor $a=0.5$ and matter dominated era, what was the size of the observable universe and matter density?
Manuel's user avatar
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Does expanding universe length units also expand? [duplicate]

Information I have read an explanation about the expanding universe which represents our universe as a metal plate that tends to expand simultaneously in all directions while being heated. One can ...
Igor's user avatar
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How a star can move out from visible universe? [duplicate]

We see only stars whose light is not older than the age of universe. This is understood. If a star is too far away that currently we don't see it then possibly we will see it later, then the age of ...
kludg's user avatar
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Hubble expansion across time dimension?

The expansion of the universe, as described by the Hubble's law, refers to the expansion of space itself. This expansion is observed in the three spatial dimensions (length, width, and height), rather ...
Mhd Afz's user avatar
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Big Bang and where do we stand relative to it? [duplicate]

Maybe dumb question, not sure, but I would need some help here to understand. https://theglobestalk.com/james-webb-telescope-see-back-in-time/ So according to physics we can look back in time ...
Panagiotis Bougioukos's user avatar
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Size of the observable universe and CMBR [duplicate]

Why can we detect the Microwave Background radiation (the earliest light that we can see) and not the light from stars beyond the observable universe? Should not the microwave radiation also be ...
Christopher Cruickshank's user avatar
18 votes
5 answers
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How do telescopes see many billion light years distant object in our universe?

How do telescopes see many billion light years distant object in our universe? As an individual with limited expertise in the field of astronomy, my current understanding suggests that the observation ...
Sazzad Hissain Khan's user avatar
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Minimum Observable Offset Frequency in Leeson's Formula

It has been argued in some publications that the age of the universe represents a lower limit on observable frequencies (in their paper, corresponding to a value of $10^{-17}$ Hz). The authors do not ...
CuriousDroid's user avatar
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Can we infer the size of the whole universe from its expansion rate? [closed]

If the universe inflated to 100 billion km in its first second, that suggests only 1/160,000 of it was observable from any point at that moment. The expansion rate slowed after that, of course, but ...
Doradus's user avatar
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Observational status of Sciama's hypothesis

I have always taken the existence of inertia more or less for granted, as an observational fact that does not require explanation. But on reflection this is an unscientific attitude, and perhaps there ...
Martin C.'s user avatar
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2 votes
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Why can infinite quantities not be shown in an experiment or observed in physics?

To modern physicists knowledge, there are no truly infinite quantities that can be shown with an experiment or observation. Time is not infinite, it had a beginning. Matter and energy is finite (...
Clockwork's user avatar
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Does only the space region movement due universe expansion affect the possibility of viewing a star?

My question is maybe simple and easy to answer because the intrinsic speed of the star affects only the final frequency of observed light but are there any tricks about the combination of object ...
jbradvi9's user avatar
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Are there any binary red supergiants?

I wonder if we ever have identified or observed a pair of binary stars (red supergiants). And I also wonder what would happen if they exploded, (theoretically) as we havent observed it. Also, would ...
schrodingerscat's user avatar
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1 answer
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Light emitted at $t = 0$

So solving for the scale factor from the Friedmann equation we can then use it to calculate proper distance via $$d_p(t_o) = c \int_{t_e}^{t_o} \frac{dt}{a(t)}$$ For a particular universe $a(t)$ is ...
ABC's user avatar
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3 answers
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If the future already exists, why can't we travel to it?

since time is actually distance, and if the future already exists, why wouldn't we be able to travel to the future? I understand humanity will never be able to, even if it was possible because our ...
Wyatt's user avatar
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1 answer
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When will the particle horizon reach its limit of 63 billion light years?

In the far future, the particle horizon will reach 63 Gly,so when will that time? Just like today's universe time is 13.82 Gyr and the particle horizon is 46.5 Gly, how many years will the particle ...
peter pan's user avatar
-4 votes
1 answer
120 views

Cosmos at minimum 250x bigger than our observable Universe, so why then the need for a Big Bang?

Please correct me If I'm wrong but does not the BB only refers to our light speed limited observable Universe (OU) from our home position? Also it is estimated that the Cosmos is minimum 250 times the ...
Markoul11's user avatar
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3 votes
1 answer
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Error Estimate For the Mass of the Observable Universe?

Recent interest in what is called "The Hubble Tension" brought to the fore potentially conflicting error estimates for $H_0$ as illustrated in be below diagram. I've been looking for any ...
James Bowery's user avatar
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Is there impedance when there is nothing else?

With the exception of gravitational waves, all the information about the universe we gained from electro magnetic waves which we agree to run at the speed of light for all the spectrum from far ...
Erna's user avatar
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4 votes
1 answer
661 views

Why don't we see the big bang?

excuse my understanding, my brain is melting. So I understand that pictures of far away objects is like viewing the past. and I think I pretty much get that the big bang was in every direction since ...
Aequitas's user avatar
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Mean distance of objects of specific size in a given volume of space - visualization of planetary systems in the universe

This has been answered in this forum before in some ways I realize, but I have a rusty ability in calculus so was unable to make use of formulas!. Would love the help! I have been talking to my 11 ...
JHouse's user avatar
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-1 votes
2 answers
114 views

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 ...
Markoul11's user avatar
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1 vote
2 answers
153 views

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 ...
Varol Cavdar's user avatar
2 votes
5 answers
3k views

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 ...
Caleb's user avatar
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1 vote
0 answers
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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 ...
matthias_buehlmann's user avatar
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1 answer
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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 ...
Jonathan's user avatar
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0 answers
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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 ...
Arman Armenpress's user avatar
1 vote
2 answers
79 views

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 ...
Арман Гаспарян's user avatar
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1 answer
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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 ...
user avatar
2 votes
1 answer
155 views

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 ...
Jack Mace's user avatar
3 votes
1 answer
619 views

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 ...
buiud's user avatar
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2 votes
2 answers
112 views

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 ...
Brian Woodward's user avatar
1 vote
2 answers
307 views

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 ...
daniel.sedlacek's user avatar
9 votes
3 answers
2k views

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 ...
Brondahl's user avatar
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4 answers
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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 ...
mae's user avatar
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1 vote
1 answer
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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 ...
Krešimir Bradvica's user avatar
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1 answer
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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 ...
Adam Herbst's user avatar
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-1 votes
2 answers
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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 ...
porton's user avatar
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2 votes
1 answer
414 views

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 ...
Number File's user avatar
1 vote
0 answers
52 views

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 ...
Arman Armenpress's user avatar
1 vote
0 answers
40 views

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 ...
Jn Rmro's user avatar
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