# Tag Info

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The Wikipedia diagram is giving the breakdown by mass not by volume. Baryonic/leptonic (i.e. non-dark) matter is only about 5% of all matter and of that four fifths of it is in the form of free hydrogen and helium. Of the remaining 1% about half is neutrinos or heavy elements. That means only 0.5% of the mass/energy in the universe is in stars.

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The answer to (2) is simply that no-one knows, and further that it's unlikely we will ever know. It's impossible to prove that the universe is infinite, but it's just possible we might prove it closed and therefore finite if the length scale is around the size of the currently observable universe. The paper Topology of the Universe: Theory and Observations ...

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In SR, there are global inertial reference frames and, in this context, no object moving with speed less than c in one reference frame moves with speed equal to or greater than c in any reference frame. But, in GR, in a curved spacetime, there are no global inertial reference frames. Instead, there are local inertial reference frames. We say ...

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A better way to think of it would be that space is being added to the universe; the universe is not "moving". Consider the case of two stars. The expansion of the universe doesn't cause the stars to "move" away from each other. Rather, space is created between the stars, resulting in them being further apart. This consequently, doesn't violate general ...

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Fundamentally, the misconception here is that something that is expanding must be finite. This is simply not true. When we say the universe is expanding, we mean the distance between two stationary observers, sitting still as best they can, grows over time. But it is entirely possible for an infinite thing to have this property. Imagine the real number ...

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There are various theories about how the matter anti-matter asymmetry arose. See this search for lots of related questions. It's generally believed that the Big Bang formed almost equal quantities of matter and anti-matter, but there was a very small inbalance i.e. there was slightly more matter. The anti-matter all annihilated with matter to leave normal ...

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Why questions ultimately in physics have the answer "because". What physics does is to model mathematically from existing observations the observed universe, in an ideal case a theory of everything towards which physics aims. The models are validated by predictions for new unrecorded at the time the model appeared, phenomena. The mathematics of the ...

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It's important to distinguish between time and the flow of time. In any universe we need four numbers to uniquely identify a spacetime point, and we conventionally choose one of the coordinates to be time and the other three space so the location of a spacetime point is given by $(t, x, y, z)$. Assuming the universe doesn't hit a singularity then for every ...

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This going to be a rather approximate answer because it involves lots of estimated quantities like the current density of matter and the value of the cosmological constant. The second Friedmann equation tells us: $$\frac{\ddot{a}}{a} = \frac{-4\pi G}{3}\left( \rho + \frac{3p}{c^2} \right) + \frac{\Lambda c^2}{3}$$ It's conventional to take $a = 1$ at ...

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If you are sincerely asking about the fate of spaceship departing the solar system, then you are asking a question about the standard model of cosmology. Instead of worrying about relativistic effects we'll concern ourselves with a photon that leaves our galaxy in a direction so that it hits nothing, and just keeps traveling. Faster than any spacecraft that ...

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In Special Theory of Relativity, all inertial reference frames are able to communicate with each other to share their results. And, they all agree with the value of c and related conclusions. In General Theory of Relativity, it's not always true. There are situations when two inertial reference frames may not be able to communicate with each other. In such ...

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The actual question in this question, is a good physics question. Freely interpreted, it basically asks if SR effects, in particular time-ordering of spacelike separated events, make it difficult or impossible to simulate physics. The answer to that is no. An "external" Simulator (be it a particle physicist or the hypothetical people simulating our ...

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We do have such structure in place: First, anything beyond the cosmological event horizon is effectively part of a different universe. An extension of that idea is the inflationary multiverse with bubble universes that can even have varying physical laws due to differently broken symmetries. String theory in turn adds its own flavour to that idea via the ...

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We model spacetime as a manifold and a metric. Broadly, the manifold gives us the dimensionality and connectivity while the metric provides a method of specifying distances. The equations of General Relativity allow us to calculate the metric from the stress-energy tensor (or vice versa if you're Miguel Alcubierre). The point that jinawee is making in his ...

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The (nearby) "separation between objects" you are referring to is the space-time metric. A metric in cosmology describes the expansion of space on large angular scales (low $\ell$ on the angular power spectrum of the universe). Without going into the mathematics, the expansion of space is driven by cosmic inflation, and is affected by things the amount and ...

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There is a three dimensional shell of galaxies (none currently observed) that have a gravitational redshift relative to us that is consistent with a relative motion at the speed of light. If you moved to any other galaxy in the universe, there is a very high probability that they would observe a different such bubble. If you look sufficiently far into ...

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Time is relative. When it comes to Time Dilation, you actually see dilated time of another observer. So, your own time flow won't get frozen in any case. Hypothetically, you can see another one's time frozen if she is traveling at speed of light (time dilation by speed) or she is at event horizon of Black Holes (gravitational time dilation). Unfortunately, ...

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According to the Deep Astronomy webpage on the HUDF 3D, These galaxies, while standing absolutely still, are racing away from us, in some cases faster than the speed of light. The spacetime between us and everything else grows larger by the minute, pushing the galaxies in this image to a distance of over 47 billion light years. So if the image is 47 ...

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Finally the bottom line of question is, can we experimentally prove that energy, total energy is actually conserved ? (a Yes answer requires a detailed experiment with complete conservation and no loopholes) Elementary particle physicists have been doing this for more than sixty years. Conservation of energy is one of the main constraints that built ...

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