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Questions tagged [universe]

The universe refers to the cosmos; all of space-time and that which exists as part of it. Alternatively, it can refer to the observable universe, which only contains the part we can see. Questions tagged with this should ask about physics at scales the size of the universe or specific properties of ...

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Number of stars in the universe vs. number of sand grains on earth

Is there an approximate-based proof that there are more stars (not including planets, although $N_\text{planets} = C \times N_{\text{stars}}$ thus $N_{\text{planets}}$ is irrelevant. Let's leave any ...
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27 views

Is Time Cyclic? [closed]

Answering Unanswered questions about Physics, Universe and Time. Your comments are welcome with derivations please. By theory of relativity, if earth is orbiting sun for the last million years ...
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36 views

A universe from quantum fluctuations [duplicate]

I'm reading the book of Lawrence Krauss a Universe from nothing, I'm asking how can quantum fluctuations only creates all of these galaxies, blackholes, dark energy etc I heard of something called ...
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42 views

Would it be possible that a deterministic be able to provide explanation to all the phenomena in the universe? [duplicate]

I'm not a physicist but am extremely enthusiastic about physics. My school of thought is more like Einstein and Newton (just mentioning these two since they are pretty famous) meaning that I believe ...
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1answer
43 views

Can holography be applied to every model?

Can all models have a holographic representation? I was talking with a physicist and he told me: "Holography is not a property you can just apply to any model of the universe willy-nilly. It either ...
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32 views

Did Big Bang really happen? [duplicate]

We know, Energy and matter can not be destroyed /created. Then how come something/some energy come out of nothing as Big Bang? This contradicts the conservation law.
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1answer
46 views

How is distance between galaxies and rate of expansion of universe calculated?

I have recently read in book 'Theory Of Everything' that rate of expansion of the universe is changing 5%-10% per 1000 million years, I would like to know how such calculation has been made. Edit: I ...
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1answer
69 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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1answer
101 views

Is the Universal Wavefunction a sensible ontological concept?

It seems to me that several quantum interpretations rely on the idea that there exists a wavefunction that completely specifies the state of the universe. Of these, the Many Worlds Interpretation is ...
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1answer
39 views

General question about dimensionality of the universe [duplicate]

I am a high school math teacher with some basic physic/lay person knowledge. Each year after the AP test, I show the movies Flatland and Sphereland to my calculus classes and talk about the ...
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1answer
64 views

Can the universe build itself back together after the Heat Death? [closed]

We all know that due to the 2nd Law of Thermodynamics the universe after a certain long period of time will eventually have all its energy spread out homogeneously and reach a Heat Death where nothing ...
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1answer
40 views

Do we have enough time for understanding space? [closed]

May be there more big bangs out there... while studding about space i some times think about a bacteria living on my body that may have a life span of 5 seconds if it has logical understating even ...
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1answer
53 views

Can somebody tell me what was the size of the universe and the expansion rate at the big bang? [duplicate]

I want to know the exact numbers and preferably the calculations that gives the size of the universe at the Big Bang and the initial rate of expansion at the moment of big bang. Does somebody know ...
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23 views

Universal Entropy [duplicate]

I’m a first-time poster... Could entropy possibly be the reason the universe is expanding and even why its expansion is accelerating? If space is not surrounded by anything at all, it would seem to ...
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1answer
26 views

What does the matter density in density parameter mean?

When we write $$\Omega_m = \frac{\rho_M}{\rho_{\mathrm{crit,o}}}$$ what density does $\rho_M$ refer to? As in, suppose our universe was made with just hydrogen. Then would $\rho_M$ mean just the ...
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3answers
56 views

Travelling at lights speed of light as universe expanding

I have read that the object, the light of which is reached us in 13.8 billion years is actually about 46 billion light years away from us now, due to the expanding Universe. Lets assume that we ...
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96 views

What is the definition of gravitational horizon $R_h$ in the $R_h=ct $ universe?

Melia's $R_h=ct $ universe proposal suggests that our gravitational horizon $R_h(t)$ should be equal to $ct$ for all cosmic time $t$, not just its present value $t_0$: "several unpalatable ...
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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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3answers
53 views

Time dilation and The Big Crunch [closed]

When/if the universe collapses in onto itself as described by the big crunch theory, would time slow down due to time dilation (relative to our experience) as all matter gets pushed together, or do I ...
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18 views

How the matter and radiation densities of universe can be calculated? [closed]

Barbara Ryden gave somehow detailed and intuitive calculation in Article 5.1 of Introduction to Cosmology but skipped detail on some values. I don't know, it's good or not but I'm adding snaps from ...
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2answers
81 views

Is the universe ~14 billion years old or that's the farthest photons which reached Earth?

I know that the universe: It's around 13.772 billion years old It expands But it's not clear to me if this is not merely the age of the farthest known photons which reached Earth.
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1answer
85 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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2answers
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Can we simulate whole universe with infinite size of RAM and modern computer? [duplicate]

If modern computers are universal turing machines and C language is turing complete, theoretically we could calculate and simulate every problem in the universe then theoritically we can simulate ...
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1answer
40 views

Cause of observed galactic red shift or acceleration in expansion of the universe: velocity vs gravity.

Scientist look out at the universe and observe red shifts for galaxies that are roughly proportionate from their distance from us and attribute it to a velocity away from us. Also there is an ...
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2answers
54 views

What type of fields existed in early universe?

In quantum field theory we associate field to every particle. So how many elementary fields exits in nature? Why are fields associated to particles different from fields associated to fundamental ...
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0answers
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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2answers
67 views

How do we know the age of Cosmic microwave background? [duplicate]

How do scientists know the age of the CMB? And how does it correlate to the age of universe?
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18 views

How can we be sure if Universe has exploded with big bang? Can't it be implosion? [duplicate]

Why do we say the universe has exploded in the event of big bang? When we talk about big bang there was no concept of outside with the absence of space, so should’t the universe be imploded into the ...
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2answers
88 views

Is the speed of light in the Universe always $c$?

Every one here knows that $c=(ε'×μ')^{-1/2}$. But what if ε' and μ' are different values in other parts of the visible universe. Maybe near the center of a Quasar galaxy or even a Neutron star?
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28 views

How can the universe expand faster than the speed of light? [duplicate]

In Neil deGrasse Tyson's Astrophysics for People in a Hurry, he writes: By now, one second of time has passed. * The universe has grown to a few light-years across... If nothing can move ...
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1answer
52 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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0answers
18 views

Is baryon asymmetry universal? [duplicate]

Baryon asymmetry takes it place on the Earth: it is almost impossible to find antimatter on the Earth, but matter occurs here and there. We ourselves are composed of matter and interact with matter to ...
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2answers
105 views

Will everything eventually be a single black hole? [duplicate]

Given the idea that gravity is always a factor (even though greater distances are excessively tiny), would everything eventually end up forming a single black hole? My thinking is that, with infinite ...
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0answers
32 views

Friedmann Article on Einstein static universe

This is a reference request. Does anyone of you know where to find that article, in which Friedmann contests the static model of the universe proposed by Einstein? I know the information are not ...
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2answers
128 views

Experimental proof of space expansion

We know the universe expands with the space expansion. This follows from the FLRW solution that nicely explains the observed Hubble flow. (Since any existing acceleration is extremely small, let's ...
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2answers
90 views

Is the space between galaxies that are close to each other contracting? [closed]

The Milky Way and Andromeda are moving towards each other as indicated by blueshift. Are they just moving towards each other through space or the space between them contracting?
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50 views

Limits of the Universe: how can be sure about the size of unobservable Universe? [duplicate]

It is established by different observations a 13.4 Gyr and 93 Glyr diameter Universe. However, how can be sure about the size of the Universe if there are portions of it we can not see? Is due to ...
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1answer
67 views

Why universe is so perfectly made for us? [duplicate]

I am very eager to learn that, "Why universe is so perfectly made for us?" Or our universe is just stable by chance?
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1answer
44 views

How to handle electron-electron or proton-proton repulsion in Cosmology?

In Scott Dodleson's book, he considers that baryons are all tightly coupled. I know this can be proved by calculating the interaction time scales and comparing with Hubble's Expansion time scale. ...
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17 views

Is time moving or we? [duplicate]

We know that time moves in one direction and causes entropy,but what if we are in an enclosed system and instead of time moving through it,we ,a universe is moving through a stationary substance time ...
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1answer
102 views

Is there any way to tell if the Big Bang happened everywhere or just in some regions of the entire universe?

On large scales, matter seems to spread uniformly in our observable universe so we think the Big Bang happened everywhere in the observable universe. Is there any way to tell if the Big Bang happened ...
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2answers
96 views

How do we know the universe is expanding and wasn't already infinite?

From what I know, the decision that the universe was expanding came from the discovery of the redshift and that other celestial objects, such as galaxies, were moving away from us; the farther they ...
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1answer
74 views

Alexander Vilenkin - The Universe probably had a beginning [duplicate]

Is it true that the Universe can not have an eternal past? It seems like Alexander Vilenkin tries to argue this? Does this imply that the Universe had a beginning? Link to his publication regarding ...
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2answers
88 views

Is there a mathmatical theory to explain all physical phenomena? [duplicate]

Does anybody know of any physicists or mathematicians who have put forward a theory to everything, for which this mathematical theory would explain all physical phenomena in the universe in one ...
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1answer
230 views

Can I avoid spaghettification with a spin?

I am aware of spaghettification effect caused by entering a black hole, as well as fundamental physics of an object in space, so here is my theoretical construct: if I place an object in space where ...
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2answers
74 views

When were each of our “ultimate fate of the universe” theories widely accepted?

I'm trying to piece together a sort of timeline for the prevailing "ultimate fate of the universe" theories, specifically from the dawn of the twentieth century through today. I know the Big Bang ...
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1answer
52 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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39 views

Is there an end to the know universe?

The second law of thermodynamics states that the entropy of the system, in this case the entire universe (everything), would always increase. It is also true that nature sometimes brings order in the ...
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1answer
63 views

Information loss in black holes

Doesn't quantum collapse or decoherence mean that information is being lost all the time? So why is the loss of information in black holes such a big deal?
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3answers
126 views

Heisenberg's uncertainty principle and shape of universe

On a TV program with some well-known astrophysicists they sad that the effect of Heisenberg's uncertainty principle shortly after the Big-Bang made matter (normal & dark) expand in a non-...