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65 votes
Accepted

Shouldn't very very distant objects appear magnified?

Yes. And they are! This is called the "Angular diameter distance turnaround" (or turnover). In the usual model for expansion $\Lambda CDM$, it is at a redshift of about 1.5 or about 15 ...
James K's user avatar
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56 votes

Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?

Ultimately, the presence of invisible mass is the most straightforward explanation for what we're seeing. We're aware that we could be wrong, but if we are, we're wrong about some of the fundamentals ...
Darth Pseudonym's user avatar
38 votes

How do we know the laws of physics are the same throughout the universe?

Let's start in the middle: What is the furthest radius we can prove from earth, with absolute certaintity, that the laws of physics do not vary? Zero. Proofs are found in mathematics and court ...
Alex's user avatar
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38 votes
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Why didn't the Big Bang produce heavier elements?

I think that your thought process is flawed in that you assume that by drastically increasing the temperature you are guaranteed to get heavy elements. As odd as this may sound, this isn't the case (...
zephyr's user avatar
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34 votes

Why doesn't dark matter clump strongly in the center of galaxies, since it doesn't feel either radiation pressure or the Pauli exclusion effect?

The reason is the fact that dark matter is non collisional. The dark matter particles interact only gravitationally, they feel no pressure, right, but they also feel no drag! No drag, no friction, ...
Prallax's user avatar
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33 votes
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Is the universe considered to be flat?

I think the reason you're suffering from conflicting sources is that you're mixing both new and old, out-of-date pieces of information. First off, the book you cited was published in 2001 - 15 years ...
zephyr's user avatar
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28 votes
Accepted

Why was the Cosmic Microwave Background (CMB) released at a blackbody temperature of 3000K rather than 30,000K?

The CMB is produced as the ionisation fraction of hydrogen falls from a high value to a very small value. Contrary to what is written in the Quora answer you may have been misled by, this happens at a ...
ProfRob's user avatar
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28 votes

Is there physical evidence to distinguish between the expansion of space and an anthropocentric universe?

I'm looking for some kind of observable evidence (that has been observed, or could be observed in the future) that could falsify one or the other theory. You seem to be requiring one (very high) ...
StephenG - Help Ukraine's user avatar
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 ...
ProfRob's user avatar
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26 votes
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What's the issue with Olbers' paradox?

Olber's paradox is - as you state - the phenomenon, that the night sky is dark, but would have to be bright as the sun if the universe was infinite and infinitely old. The unspoken assumption here is ...
planetmaker's user avatar
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26 votes

Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?

Let me use an analogy as an example of being able to say something about a thing we never see. I can fairly confidently say that Zinn exist. Why can I say that? I'...
slebetman's user avatar
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23 votes
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How do we know the laws of physics are the same throughout the universe?

Nothing can be proved "with absolute certainty"; that is not how science works. We adopt a working hypothesis that the constants of nature are exactly that; both constant in time and space. Then we ...
ProfRob's user avatar
  • 153k
22 votes

Does the universe have a celestial north?

No. There is no prefered direction of the universe. The universe is homogenous and isotropic, on the large scale, to the best of our knowledge. This means that there is no "special" ...
James K's user avatar
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21 votes
Accepted

When will the number of stars be a maximum?

TL; DR Somewhere between now and a few hundred billion years time. (For a co-moving volume) Now read on. If stellar remnants are included, then the answer is very far in the future indeed, if and ...
ProfRob's user avatar
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21 votes

Why isn't the CMB blurred by the blackbodies emitted in the time after atoms first formed?

It is. They're called galaxies and stars and we see them against the cosmic microwave background. However, they aren't (all) cooling because of the energy sources they contain. But perhaps that isn't ...
ProfRob's user avatar
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21 votes
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What equation tells you how far in space you can go from a point and return?

Preparation Let $a(t)$ be the cosmic expansion factor. Let $x$ be a comoving coordinate, so two objects (like galaxies) that are separating solely due to the expansion of the universe have a constant ...
Sten's user avatar
  • 4,693
20 votes

Why is WR124 not brighter?

It's 7.7 times the absolute temperature of the Sun and if it is 12 times the radius. The luminosity will therefore be $(7.7)^4\times 12^2 = 515000$ times that of the Sun. Close enough, given you haven'...
ProfRob's user avatar
  • 153k
19 votes

Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?

"Is there something wrong with the way I'm thinking about this?" Fundamentally, yes, but it's a matter of the philosophy of science as much as the science itself. The question makes the ...
ScienceSnake's user avatar
  • 1,040
18 votes
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Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?

The friends-of-friends (FOF) algorithm (Huchra & Geller 1982; Press & Davis 1982, Davis et al. 1985) for finding groups of particles can be used for various numerical problems, not just ...
pela's user avatar
  • 38.5k
18 votes
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In 2023, is space still thought to be "filled with a network of wormholes"?

It was never thought to be the case. A fuller quote: So, by this analogy a black hole is a kind of bottomless pit. What would happen if you fell in? Assuming you could survive the gravitational ...
James K's user avatar
  • 123k
17 votes
Accepted

Which came first: Galaxies <=> Stars <=> Planets?

The structure we see in the Universe has formed from the gravitational collapse of the matter that was once an almost smooth density field of gas ("baryons") and dark matter$^1$. The word &...
pela's user avatar
  • 38.5k
17 votes
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Understanding The Turnover Point of Angular Diameter Distance

On the one hand an object spans a smaller angle the farther away it is, as expected. On the other hand, due to the expansion of the Universe and the finite speed of light, very distant objects were ...
pela's user avatar
  • 38.5k
17 votes

What equation tells you how far in space you can go from a point and return?

sten's answer is excellent and beautifully analytical; I just wanted to illustrate the journey and show that neglecting matter (and radiation) is a good approximation. Spacetime diagram To do this, I ...
pela's user avatar
  • 38.5k
17 votes

What would evidence of stars and galaxies significantly older than 13.8 billion years old look like? In what parts of space has it not been found?

Assuming the cosmological principle still holds, then we might expect that your own galaxy and those around us should contain evidence of stars/objects older than 13.8 billion years old. These might ...
ProfRob's user avatar
  • 153k
16 votes
Accepted

Latest cosmological parameters

Cosmological parameters are measured in a variety of ways, and their values will depend on which measurements you trust the most. The paper you link to (Planck Collaboration et al. 2016) with the 2015 ...
pela's user avatar
  • 38.5k
16 votes

How can gravitational lensing makes a quasar appear brighter?

The quasar gives out light in all directions. The light spreads out in space. Only a very small amount of that light would be pointed exactly in the direction of your telescope. But if a large galaxy ...
James K's user avatar
  • 123k
16 votes
Accepted

How is the Hubble constant determined from gravitational waves?

If you measure the gravitational waveform from an inspiralling binary, you can at any point measure the amplitude, instantaneous frequency and the rate of change of frequency. The last two give you ...
ProfRob's user avatar
  • 153k
16 votes

Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?

This is a slightly more technical answer that should nonetheless illustrate the concepts. Wind back the clock to 1845. You are an astronomer of that period, and you are familiar with Newton's Laws, ...
Allure's user avatar
  • 4,554
15 votes
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Why do we continue to find a discrepancy in the Hubble Constant?

To make a long story short, the measurements from Planck and the Hubble Space Telescope disagree, and the reason behind this isn't known. First, let's look at the values with the uncertainties. We ...
HDE 226868's user avatar
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