# Tag Info

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Every thesis submitted for a PhD in Cambridge is archived at the Cambridge University Library. They should be able to get you a copy (for a fee). See http://www.lib.cam.ac.uk/deptserv/manuscripts/dissertations.html

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The latest constraint on the curvature of the universe comes from the Planck Mission which indicates that the universe is very flat today.

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The following graph indicates the amount of radiation, matter (which almost is entirely made of dark matter), and dark energy vs time. We are at a point in time where dark energy has just started to dominate. For instance, if you go back to the first few years, radiation was the most abundant source of energy.

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Assuming you found a way and managed to accelerate above light speed without disintegrating, and went to the edge of the universe... I'm confident you can't go faster than light, but when it comes to the edge of the universe, I'm also confident that nobody knows any answers. However people say they do and state categorically that there is no edge. For ...

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Light speed travel is impossible, so you are asking what happens to a system when we totally ignore the system. Aside from that, the universe probably does not have an edge. The observable universe does have an edge, but you can never reach it. This is because when you move, the the edge of your observable universe moves with you.

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"Why" is never a physics question, but we can replace it with "What is time?", which is. "Time" is that which a suitable clock shows. It's not some abstract, undefinable philosophical notion that magically "makes things happen". Time in physics is a stream of steadily increasing numbers coming from suitable experiments that measure time. We call those ...

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Snap your fingers. Wait. Snap them again. Physically speaking, time is a measurement which distinguishes between these two different events (finger snaps) which happen non-simultaneously (snap, wait, snap) at the same spacial location. There is no reference frame in which these events happen simultaneously. That's the simplest way I know to define time. ...

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Red shift. is a really quick answer. whichever way we look, stuff is moving away from us. and the further away it is the faster it's moving.

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In the FLRW model which is used today normal (baryonic) matter and dark matter together add up to matter. It dilutes with the growing radius to the third power (because volume is proportional to radius³). So the ratio matter : dark matter is and was always the same, at least in the plot you showed and the model that was used there. Edit: Maybe the 5.25 to ...

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The ratio of dark to baryonic matter is 5.25 in the first diagram and 5 in the second diagram, but I don't think the difference is significant. We don't know the densities with absolute certainty, especially near the Big Bang, and the small difference between the ratios is probably just down to the uncertainties in the densities. We would expect the ratio ...

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The concept of the expansion of the universe is hard to get your head around. We describe the universe is infinitely large, and it is expending into itself. So, there is no outside, You can't leave the cosmos. If you could find the edge of the universe and exit through that edge, you would re-enter the universe from the other side. Edges of the universe are ...

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I've heard that some physicists think that the net energy of the universe is zero. Me too. They talk about gravitational energy being negative. But see Einstein talking about gravitataionl field energy here. It's positive. For this to happen, I would assume that the negative gravitational energy of a body ought to cancel out its rest energy. That's what ...

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The ultimate fate of the universe (and here I'm taling about things on cosmic scales not what happens to stars and galaxies etc.) depends on the equation of state of the material within it. In cosmology the equation of state is represented by a dimensionless number that is the ratio of the pressure to the (energy) density. i.e. $$w = \frac{P}{\rho}$$ and ...

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This part of Lawrence Krauss' brilliant "A Universe from Nothing" lecture describes the fate of an expanding universe quite well. Long story short, because the expansion of the universe is accelerating, in a hundred billion years or so the rate will exceed the speed of light (this doesn't require objects to move through space faster than light, that's ...

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I believe that the ever expanding universe is the most popular theory as of now. But what is making the universe accelerate? We believe it has to do with Dark Matter and Dark Energy but no conclusive research has been done yet. So in general, we just don't know what will be the end fate of the universe because there is too much unknown about what is ...

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The size of atoms is determined by the strength of the electromagnetic force, the mass of the electron and some other constants like the value of Planck's constant. If the size of atoms was changing it means that one or more of these constants must be changing. The trouble is that these fundamental constants crop up all over the place in physics, and if ...

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In your model, of everything stretching, there would be no way to observe any stretch. We would live in static dimensions as far as we are concerned. It is our observations of the way the stars, galaxies and clusters of galaxies behave that has led to the idea of an expanding universe : Hubble inferred the recession velocity of the objects from their ...

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We can predict what is more probable to happen. Our mathematical models do not describe the nature exactly. For example, consider angular velocity, defined as $\omega=2\pi/T$, where $T$ is the period. We can only estimate the value of $\omega$ of any rotary system because we don't know the exact value of $\pi$.

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Assuming inter-universal travel is possible, can a perfectly mathematically describable universe exist inside a multiverse? I would actually say no. Assuming inter-universal travel is possible, there may be sources of randomness coming in from other universes. Namely, the multiverse could influence the universe, making it unpredictable. For example, ...

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How can one predict the future of such a universe when inter-universal travel is possible and the multiverse is not perfectly mathematically describable? Inter-universal travel should NOT be possible if you are speaking of multiverses in the usual sense of inflationary cosmology. The idea of one universe being separate from another hinges on the idea ...

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Disclaimer: Here is my metamathematical take (but read that as a very very very recreational bunch of statements). Gödel second incompleteness theorem says that the completeness of a logical theory (i.e. that every statement is true, false or undecidable) is not provable within the theory. However, if you are willing to assume proofs that "are more complex ...

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I interpret "perfectly mathematically describable" as a universe which obeys the same physical laws as our own. The answer is yes. The standard inflationary model allows for multiple, causally disconnected regions to be inflating simultaneously, creating separate universes. There may be some differences in the physical constants of those universes, but they ...

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