42
votes
Accepted
If string theory is inconsistent with observations, why hasn't it been rejected yet?
You surely know that string theory has zillions of vacua. Most of these vacua can immediately be ruled out e.g. because they have the wrong number of macroscopic dimensions, or for similar reasons. ...
- 11k
30
votes
How does the whole universe agree on the laws of physics?
Have you perhaps mistaken the meaning of the word "law" in this context? It simply means "pattern" or "regularity". If a given property or behaviour is neither constant ...
- 47.6k
27
votes
If string theory is inconsistent with observations, why hasn't it been rejected yet?
As an experimental physicist I will answer the title. The examples of inconsistency in the question deal with many assumptions on cosmological observations and models, and are answered by @...
- 223k
21
votes
Is Big Bang Theory the only one to explain the creation of the Universe?
The big bang model$^{1}$ is an attempt to explain a host of observations that tell us how the universe evolved from its first fraction of a second onwards.
It started off by being grounded in ...
- 113k
20
votes
So Universe Expansion is accelerating, it already did with the Big Bang, so what came in-between?
The expansion of the Universe is driven by the dominant forms of energy and matter density. This is essentially an expression of a famous statement by John Wheeler (from memory, apologies if I get the ...
- 35.4k
19
votes
Is the Big Bang defined as before or after Inflation?
The Big Bang was originally defined as the zero time limit of the FLRW metric, so it's a mathematical construct and not primarily something physical. We have chosen to apply it to the zero time limit ...
- 334k
18
votes
Is the Big Bang defined as before or after Inflation?
So I've done some further research into this question and the result I found is quite surprising. There truly is no set definition. Some cosmologists will tell you (as John Rennie mentioned) to avoid ...
- 24k
18
votes
Accepted
Why was more dark energy created during the creation of the Universe than normal energy?
It is not currently known why there is more dark matter than normal matter because no one knows what dark matter is yet. Dark matter is like normal matter in that it pulls space together with gravity.
...
- 1,713
16
votes
Could quantum fluctuations spawn real matter?
The answer is no. And to be clear about this: the set of quantum fields in their least energy state, which we call the vacuum, when left to its own devices, in the absence of stuff (including ...
- 47.6k
16
votes
Accepted
Are quantum fields such as electron field fundamental?
Within the Standard model of particle physics, the most general experimentally verified model of fundamental physics (excluding gravity), there are quantum fields, such as the electron-positron field, ...
- 18.4k
14
votes
Accepted
If all the Structure in our visible Universe originated from Quantum Fluctuations, why isn't there as much Anti-Matter as Matter?
The matter-antimatter asymmetry requires the three Sakharov conditions to be satisfied. I'll summarise that link's explanation. Unfortunately, your question isn't completely solved.
The first ...
- 22.4k
13
votes
Accepted
Damhsa Theory: Can gravitational waves really affect the long term climatic evolution of Earth?
"The Damhsa Theory proposes that these
oscillations are actually gravitational waves from the
continued ramifications of the expanding universe and
that ice ages and inter-glacials such as our current
...
- 30.5k
13
votes
How does the whole universe agree on the laws of physics?
The hypothesis that the fundamental laws of physics are the same everywhere in the universe and at all times is based on empirical evidence. Detailed observations of distant galaxies confirm that ...
- 35.5k
13
votes
What would the wavelength of the Cosmic Background Gravitational Wave radiation be?
It is unlikely that we can detect gravitational waves from the Big Bang with current technology. Due to universal expansion, such waves would have very large wavelengths.
We would need interferometers ...
- 25.6k
11
votes
If string theory is inconsistent with observations, why hasn't it been rejected yet?
String theory's apparent "incompatibility" with the existence of de Sitter vacua and inflation is just a sharpening of the apparent "incompatibility" of quantum field theory, semi-...
- 3,595
11
votes
Are the unification of spacetime and the warping of space in general relativity real, or mathematical convenience?
Does the Earth really go around the Sun, or is this just a mathematical convenience? For some purposes we use a geocentric model and say "the sun rises in the East"; but creating a ...
- 6,214
11
votes
Are Laws of nature independent of time?
There are two approaches to time dependence in physical laws. One is to say that there could be different laws at different times. Another is to say that there is one "true" unchanging law, ...
- 35.5k
11
votes
Accepted
Inverse Laplacian
Let's write an arbitrary function $f:\,\Bbb R^3\mapsto\Bbb R$ as a Fourier transform:$$f(\vec{r})=(2\pi)^{-3/2}\int_{\Bbb R^3}\tilde{f}(\vec{k})e^{i\vec{k}\cdot\vec{r}}d^3\vec{k},\,f(\vec{k}):=(2\pi)^{...
- 22.4k
11
votes
Inverse Laplacian
Every Laplacian can be inverted using its Green's function. If we have
$$\nabla^2V = \rho$$
the inverse is simply
$$V(x) = \left(\nabla^2\right)^{-1} (\rho) = -\frac{1}{4\pi}\int \frac{\rho(x')}{|x-x'|...
- 6,491
10
votes
Accepted
What is meant by the term "value" of a scalar quantum field?
Your question is not specific to inflation, and really applies to any case where a bosonic quantum field behaves semiclassically due to macroscopically large occupation numbers. One very simple ...
- 3,697
10
votes
Are quantum fields such as electron field fundamental?
Quantum fields can exchange energy. So for example when an electron and a positron annihilate the energy that was in the electron/positron field is transferred to the photon field. The result is that ...
- 334k
10
votes
Are Laws of nature independent of time?
Yes, the laws of nature are independent of time.
That is, when and if we have the 'correct laws' of physics.
It's quite subjective and philosophical as physical laws can't be proved right, only ...
- 13.4k
9
votes
Accepted
What is the aim of modified GR theories?
First, there is no reason why all possible higher-order terms of the Einstein-Hilbert action could not in principle be there. At low energies, the effects of these higher-order terms would be less ...
- 1,112
9
votes
How does the whole universe agree on the laws of physics?
Let's consider an example of this violation. Consider the electrostatic attraction strength (Coulomb's constant). If we compute it and get some value $k_1$ in our labs, we might conclude that it is a ...
- 1,800
8
votes
How far has a 13.7 billion year old photon travelled
Light always travels at the speed of light when in a vacuum. Space is a pretty good vacuum.
So if it's been travelling for 13.7 billion years, then it has travelled 13.7 billion light years.
There ...
- 113k
8
votes
Accepted
What are the roles of pre-heating and re-heating in inflation?
During inflation you have the inflaton field which stays almost constant in a flat region of its potential and therefore drives the exponential expansion of the universe. Because the universe expands ...
- 685
8
votes
Accepted
What is a slow-roll field?
The most straightforward theories of inflation assume there exists some scalar field $\phi$ that permeates the Universe and drives inflation. Over time this scalar field changes, and the rate of ...
8
votes
Where were the first galaxies formed in our observable Universe, at the center or at its outskirts?
Our universe does not have a “center.” The distinguishing feature of our location is that we live here. We have excellent evidence that the universe is “isotropic,” which means the time from the Big ...
rob♦
- 74.3k
7
votes
What is chaotic about Chaotic Inflation?
Linde's model, unlike the first model proposed by Guth, assumes random initial conditions. So
Chaotic = almost arbitrary initial conditions
This implies that not all regions of space will undergo ...
- 3,750
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