# Tag Info

### 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 ...
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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 (...
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### 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, ...
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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I think the reason you're suffering from conflicting sources is that you're mixing both new and old, out-of-date pieces of information.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 ...
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### 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 ...
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### 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) ...
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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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### 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 ...
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### 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" ...
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### 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 ...
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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 ...
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### what is a Friedmann model?

The "Friedmann model" is a model of the Universe governed by the Friedmann equations, which describes how the Universe expands or contracts. These equations are a solution to Einstein's field ...
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### 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 ...
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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 ...
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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 ...
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### 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 ...
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### 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 ...
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### 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 &...
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### 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 ...
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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 ...
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### A Sky Map of Gravity - what would it look like?

The rough answer is: just like the sun makes it hard or impossible to see planets and stars during the day, it dominates the gravity sky. But there are interesting patterns there if we view the sky ...
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### Why did astronomers believe most or all stellar black holes had masses no greater than 15 solar masses?

The so-called 'mass gaps' for black holes, according to theoretical models, are between 2-5 solar masses and 50 to 150 solar masses. (Actually, I have read that there is no good theoretical reason for ...
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### What happened to the reemitted photons during recombination?

There should indeed be emission lines at the appropriately redshifted frequencies. However, they are going to be incredibly faint and diluted because the ratio of photons to baryons at the epoch of ...
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### Why do we see a cosmological redshift at all if space is not expanded in our solar system?

tl;dr Because space doesn't contract inside our Solar System. Wavelength increase is proportional to space expansion The prediction of general relativity — one of the most thoroughly tested and ...
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### Is the Universe really expanding at an increasing rate?

I don't take this at face value because we should expect more distant objects to have higher observed speeds and therefore higher observed red-shifts. That's true. That was the original Hubble ...
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### Existence of gravitons?

Gravity is a fictitious force, actually, much like centrifugal force. In a free falling frame of reference it disappears. In general relativity (GR) gravity is just a result of (differential) ...
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### Are there ways to estimate size of the "whole universe"?

tl; dr The universe is probably infinite, but if that's the case it's impossible to verify. If the universe is finite, and small enough, and the global curvature is equal to the curvature of our ...
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