54 votes
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How do scientists know that the distant parts of the universe obey the physical laws exactly as we observe around us?

We don't know in general but to the extent we can measure, the laws seem to be the same, even if conditions are not. For example radioactive decay: We know how fast various elements decay, and we can ...
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45 votes
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What is the hottest thing in the universe?

Energetic neutrinos have been observed from the core of a supernova (SN 1987A). The inferred temperature at the "neutrinosphere" is about 4 MeV (equivalent to 50 billion K - ($5\times 10^{10}$ K, ...
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33 votes
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How much larger will the "observable by us" universe be when JWST becomes operational?

tl;dr Conservatively 10%, realistically 25%, optimistically 60%. I assume that by the "observable-by-us Universe", you mean not the theoretically observable Universe, which is given by the ...
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27 votes
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Does the recent news of "ten times more galaxies" imply that there is correspondingly less dark matter?

All Conselice et al. (2016) appear to suggest is that when you look at something like the Hubble deep field, there are many faint (and presumably low mass) galaxies that are not seen. This has ...
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24 votes
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Why can we observe the Cosmic Microwave Background no matter the direction we look?

Until the Universe was 380,000 years old, it was filled with a gas of protons an electrons. There was also radiation, in thermal equilibrium with the matter, and because it was so hot, the protons and ...
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24 votes

What is the hottest thing in the universe?

Note that while we haven't observed anything even close, there is a theorized Absolute Hot along the lines of absolute zero. Its theorized value is ~ $1.416 \cdot 10^{32}$ Kelvin. Above this ...
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22 votes

Are there any galaxies which fell out of sight horizon due to cosmic expansion?

No. In fact the opposite is the case. (See the last paragraph for an intuitive explanation.) It is a common misbelief that galaxies receding faster than the speed of light are not visible to us. ...
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21 votes

Why can't we see distant galaxies with the naked eye?

Surely if you stared long enough, the light from them would eventually hit your eye? Collecting light over a long span of time is how telescopes can see very dim objects. The human visual system ...
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14 votes

Why can't we see distant galaxies with the naked eye?

Not at all a dumb question, but actually you can see distant galaxies with the naked eye. From the northern hemisphere, the Andromeda Galaxy, our biggest neighboring galaxy, is visible if you know ...
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12 votes
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How can I see a nebula?

Yes, indeed! Many nebulae are visible from Earth in a small and cheap telescope, and even to the naked eye (if you are standing in a sufficiently dark place). In fact, yesterday I was watching the ...
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12 votes
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Can we see the Big Bang happen if we look far enough?

No. The furthest we can see is the cosmic microwave background radiation (CMB). Early on (after the big bang), matter was fully ionised and the electrons frequently interacted with the photons. That ...
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12 votes

Logically, how can the universe be infinite in size?

Something infinite can expand. Consider an infinite length of elastic. There are (infinitely) beads attached to it at 1m gaps. You might label one of the beads "0", then the next one is &...
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11 votes

Photon Paradox?

And that is why you don't do the calculations in a frame that is moving at lightspeed. If you have two observers that are moving relative to each other you can use the Lorentz transformation to ...
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11 votes

Does the age of the universe take into account General Relativity / Special Relativity?

This is a confusing question - your title mentions GR, but of course the age of the universe is entirely derived as a result of using GR to solve for the dynamics of the expanding universe. The text ...
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11 votes
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Does the age of the universe take into account General Relativity / Special Relativity?

Rob Jeffries gives a good response to this question, but I wanted to go through the basic outline of how the age of the universe is calculated, just so you can see how it works more or less. Be warned ...
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11 votes
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If we could fly off edge of observable universe what would happen?

The 'edge' of the observable universe is as much a edge as is the 'edge' of how far you can look from the roof of your house: none at all, it's just a limit to our vision. We can never reach this edge ...
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9 votes
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Is there a physical limit to how far we can go?

Your 3 points are spot on. There is a finite number of galaxies we could theoretically reach for the reasons you say. The further away the distant galaxy the greater the expansion of space between ...
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9 votes
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Where is the North of the Universe

Sort of... There is a system called the International Celestial Reference System (ICRS) which has center at the Solar System Barycenter (normally inside the Sun but not the same as the Sun's center) ...
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9 votes
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Does the universe curve in on itself?

In the currently dominant theories, the Universe is basically the same everywhere, if you look on a large enough scale. There may not be a furthest star from Earth at all (the universe may be infinite)...
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9 votes

How is the gravitational effect of galaxies outside of the visible universe on galaxies within the visible universe currently modeled?

We don't know… We don't know how much Universe there is outside our observable Universe. The observable Universe seems to have a "flat" geometry (in the 3D sense, not in 2D). If it really is ...
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9 votes

Is the age of the Universe really 13.8 billion years?

The age of the universe is not calculated based on the size of the visible universe. The age of the universe is being calculated based on the fact that the laws of nature have no direction. This means ...
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9 votes

Rotation of far away and near by galaxies

It depends a bit on what you means by "far away" and "the same", but: Galaxy formation Galaxies form from collapsing and colliding clouds of gas and dark matter in the early ...
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9 votes
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How is observable matter distributed in the universe?

I think the following is a fair summary. It's based on a fairly old study by Fukugita & Peebles (2004) but the numbers are quite reasonable. Your guess about stars dominating is way off. Most of ...
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8 votes

Are there any galaxies which fell out of sight horizon due to cosmic expansion?

As time passes, there are galaxies that are currently not in the observable universe which will become observable But this is not a sudden winking on. Instead, over hundreds of millions of years we ...
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8 votes
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How long would it take to reach the edge of the reachable universe?

Jonathan's answer is essentially correct, but as Rob Jeffries comments, he doesn't take into account that the Universe is expanding during the journey. The edge of the observable Universe is 47 ...
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8 votes

How do scientists know that the distant parts of the universe obey the physical laws exactly as we observe around us?

See also: Do the laws of physics work everywhere in the universe? Noether's theorem, in the context of this question, states that: If the laws of physics do not vary with position, then linear ...
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8 votes
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Why can't we point the centre of the universe from inflation graph we see?

That diagram does not depict the entire universe. At most, it depicts the history of what is now our observable universe (specifically, a 2D slice through it), with us at the center only because we're ...
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8 votes
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Why would a quantity like the 'Hubble contrast' be squared, then have its square root taken?

The brackets refer to the average, so $\left< x^2 \right>^{1/2}$ is the root-mean-square (RMS) of $x$. That is the square root of the mean (or average) of the square of multiple $x$s. The RMS ...
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  • 555
7 votes
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What fraction of galaxies in the observable universe have we actually observed?

There are around 2 trillion galaxies in the currently observable universe according to the latest estimates, obtained by integrating theoretical galaxy stellar mass functions above $10^{6} M_{\odot}$ ...
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7 votes
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How thick is the cosmic microwave background, including the part we cannot see within the observable universe?

If I understand you correctly, you want to know the distance from the point from which we observe the CMB, to the edge of the observable Universe. During inflation, the observable Universe expanded ...
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