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 the universe

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Schroedinger equation and an infinite universe

Given that the Schroedinger equation states that a particle can be found an infinite distance away from its "center" and the universe is infinite, why don't we find infinite particles at any given ...
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How does an expanding universe and a constant-density vacuum energy not violate energy conservation? [duplicate]

As per the title. I'm probably thinking too naively in a classical way, but intuitively it seems to me that: the presence of vacuum energy at fixed density throughout all space and, an expanding ...
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According to Hartle-Hawking state, could we build a sum over all possible metrics (including non-compact ones)?

Physicists Stephen W Hawking and James B Hartle 1 proposed that the universe, in its origins, had no boundary conditions both in space and time. To do that, they proposed a sum over all compact ...
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In the big bang model did the universe reach its current size in less than one day?

I read that the Inflationary Epoch is posited to have lasted $10^{-32}s$. That seems quite quick. In the big bang model did the universe therefore essentially reach the size it is now in less than ...
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Simple explanation of expanding universe, red shift and time difference between galaxies

Given that the red shift of galaxies increases with distance, and that the light from further galaxies has taken longer to travel - then it seems that comparing red shift between galaxies needs to ...
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Does the second law of thermodynamics apply to parallel universes or higher dimensional universes? [closed]

Does the second law of thermodynamics apply to parallel universes and universes of higher dimensions?
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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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Expansion of the universe by scaling of space

The Wikipedia article on expansion of the universe states that: expansion is an intrinsic expansion whereby the scale of space itself changes (...) the metric governing the size and geometry of ...
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Did Stephen Hawking propose that the universe arised from truly nothingness?

Stephen Hawking giving a lecture describing how the universe would have arisen from his model (developed with James Hartle) or "no boundary" conditions, once said: Unlike the black hole pair ...
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If space is expanding, what is the effect on $1/r^2$?

Space is expanding, and this effect can be only detected at large scales, on the scale of galaxies and larger. If space is expanding, the expression $1/r^2$ for the law of gravity will be modified. ...
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What can we find if we went back to where the big bang happened?

Space after the big bang occurred have expanded dramatically. However at the same time the big bang happened it started to expand from that moment so what can we find before that point. For example, ...
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Increasing observable universe size using primordial gravitational vaves

Using photon decoupling, the size of the observable universe is generally evaluated as being around 45 billion light-years at current time. When, and if we can detect primordial gravitational ...
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What does $w=-1$ mean for the equation of state of the cosmological constant?

The Planck collaboration has measured $w=-1.0$ for the equation of state of the cosmological constant Lambda. Can somebody explain what this means? (1) Does $w=-1.0$ mean that Lambda was constant ...
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What does cosmological redshift say about speed of separation and why?

I've been thinking about cosmological redshift lately and in particular its effect on the apparent speed of separation. In particular, let's imagine a source which emits light pulses with a period of $...
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Determining the size of the universe to calculate its age

Recently, I started to ponder again with the question of how do we know the age of the universe. From my research, the answer is something like: "We measure the expansion rate directly with type ia ...
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(How) Can we witness the early universe in an infinite universe?

I recall witnessing a discussion with Dr. Brian Cox from the University of Manchester discussing measurements that have concluded a flat universe. He brought up the suggestion that this is perhaps ...
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Is Big Bang Theory the only one to explain the creation of the Universe?

Is Big Bang Theory the only one to explain the creation of the Universe? I have read about Big-Bang Theory explaining the creation of Universe. Is there no other theory to explain the creation of ...
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Why are the most distant galaxies “only” around 13 billion light years away?

According to Wikipedia's List of the most distant astronomical objects, the most distant galaxy (GN-z11) is estimated to be around 13.39 billion light years away from earth. However, the observable ...
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32 views

Relativistic Mechanics & Conservation of Energy [duplicate]

It is a central tenet of relativistic mechanics that there can be no such thing as an absolute frame of reference, and as such, it appears to me that it is physically impossible to measure the total ...
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Why can't we decrease the total entropy in the universe?

I have a question: why can the total entropy in the universe not be reduced? I know that an open system's entropy can decrease, and a closed system goes to the maximum entropy. But for the universe, ...
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Thermal equilibrium in radiation dominated universe. What drove the universe out of this equilibrium?

So in our GR lecture, we studied the $\Lambda$CMB model. And we neglected interactions between different contents of the universe and we found the following: The universe was first dominated by ...
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Conservation of energy violation [duplicate]

Is there any violation of the conservation of energy in the universe? Pls give a specific example of it using the Noether's theorem. No maths required.
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Energy density of a universe with inflation

What is the function discribing the energy density of the universe including inflation and a dark energy dominated phase?
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How does cosmic bodies set themselves up to rotate around others?

We know from newton laws of gravity how planets, moons etc rotate in orbit around other bodies but how does this really get setup when we know everything starting with a big bang? As with big bang or ...
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Why is the flatness problem a problem?

Friedmann equation states $$ H^2 \equiv \left( \frac{\dot{a}}{a} \right) = \frac{8\pi G \rho}{3} - \frac{k}{a^2} $$ which describes the evolution of the expansion of the universe. Writing $\Omega(t)...
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How to calculation 'lifetime' of inflation model [closed]

How to calculate the 'lifetime' of an inflation model? How are calculations performed with the help of a Hubble parameter for an inflation model?
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Calculation of the radius of the observable universe

Wikipedia mentions the radius of our observable universe based on the travel time of light from remote objects whose light’s travel time is equal to the age of the universe. “According to ...
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Nature of distant objects in a collapsing universe?

If we reformulate the hubble radius such that $r_H=\left|\frac{c}{H_0}\right|$ for a collapsing universe, we have that at $r_H$ objects approach us at $c$. My question is, what do we observe? This ...
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What would happen if I rotated an extremely long stick? [duplicate]

(Exclude all movements of myself) Imagine I had a straight stick that reached far out in the universe. If I rotated that stick, what would happen to the speed of the stick and its shape? (First ...
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Are elementary particles as old as the universe?

Lets take quarks for example. Are the quarks - that constitute the universe - as old as the universe? Or are particles always created? Or were they all created when the universe was born?
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Are questions about “when” an event outside the visible universe occurred meaningless?

It seems to me that the answer is yes, first of all because it is impossible in principle to use Einstein's definition of simultaneity in such cases since signals cannot pass from the event to us, and ...
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Since the CMB is the oldest thing we can see, how do we know for sure what happened before the CMB?

I keep wondering about how can we be so sure about what happened before the CMB, given that it is the oldest thing we can actually see. It seems like we are very confident about what really happened ...
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What existed before Big Bang? [duplicate]

This is one of the most famous and trending question in today's world. Well , before you answer this question I want to say that before answering this question you must know about the Big Bang Theory &...
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Is there any method to know the real age of the universe? [duplicate]

Well, I was wondering about the real age of our universe, I found that it's estimated to be $13.8\times 10^9$ years. Is it an approximation, or laws behind this age?
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Scale factor in conformal time

I'm having trouble getting the scale factor for conformal time. In de Sitter we've $$a(t)=\sinh(H_It), -\infty <t <\infty$$ and the relation: $dt=ad\eta.$ I don't see how this relation ...
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Does the Big Bang have the absolute lowest amount of entropy? [duplicate]

We know that the event of the Big Bang has low entropy (and we don't know why), but... do we know for certain (or through convincing mathematical evidence) that the event of the Big Bang indeed has ...
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Restart the Universe expansion? [closed]

Is there anything that could restart the universe expansion? Basically a black whole or something else entirely different? It seems to me that, once the Universe stops expanding, the human life ...
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Why is the Andromeda galaxy coming closer to Milky way?

If both these galaxies are on collision path because of their gravity but how could that be? They are 2.5 million light-years apart now but I imagine they are still n collision path for a long time ...
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Could something theoretically observe the first light in the Universe?

Light travels at 299,792,458 meters a second. If you look at something that is 299,792,458 meters away, you're observing light from 1 second in the past. If an observer's distance from a light source ...
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Vacuum energy density with respect to what reference frame?

Wikipedia quotes the observed vacuum energy density to be around $10^{-9}\frac{J}{m^3}$. Whatever the real energy density is, my question is, with respect to what reference frame is this being quoted?
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Expansion of space or contracting of matter? [duplicate]

Can the expansion of the Universe effect be explained by contraction of the overall matter instead? That is, all material objects are in fact contracting, due to what the universal space seems (and ...
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Does light from source one mil. light years away take more than 1 mil. years to reach us?

Because of the expansion of space itself, did light from a galaxy one million light years away actually take more than one million years to reach us? Also, if we are looking at a galaxy one billion ...
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1answer
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Laws of Physics [closed]

Here is a thought. Let us assume the there is an infinite uni-directional line of balls. And it is being jumbled up again and again, randomly. (say by 'God'!) I in my limited experience/capacity can ...
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Is the relativistic Doppler effect the only way to show that the universe is expanding?

Is there another way to show that the universe is expanding, rather then only relying on the relativistic Doppler effect?
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If energy is neither created nor destroyed, where is the extra space coming from? [duplicate]

I've been learning about many aspects of science all of my life, and have come to understand the following: Energy is neither created, nor destroyed. (Classical Physics) Space is expanding. (...
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If space expands, why does a liter of water stay a liter? [duplicate]

From observing the universe, we know that space expands. I see what that means on cosmic scales. But what does it mean on smaller scales? If I have a graduated beaker of 2 liter, and 1 liter of ...
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Galaxy is 8 times closer 13 billion years ago? [duplicate]

Based on below, How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ? I ...
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Calculate distance of galaxy 13 billion years ago [duplicate]

How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ?
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How do we know that the space metric is expanding as opposed to objects moving relative to one another?

So we observe that the Universe is expanding. We observe galaxies moving away from us and the speed/red-shift is greater for galaxies that are further away. How do we know that this observation is ...
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If everything in the universe is drifting away will We eventually be able to register things disappearing?

If everything in the universe is slowly drifting away how soon before we see signs of this?Maybe we’ve already registered things at the visible edge of the universe disappearing. If not will, it ever ...

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