# Tag Info

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This is a difficult question for many reasons. One reason is likely because most of the introductory thermodynamics textbook problems that we are familiar with from childhood do not involve gravity. To illustrate this difficulty with gravity consider, for example, this snippet from an article in the New York Times Review of Books by physicist/mathematician ...

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This is a good idea... Dark matter by definition doesn't interact electromagnetically (i.e. it has no charge). Therefore its cross section $\sigma$ for absorbing radiation and being pushed away from an accreting object is $0$, at least to first order. You could look at higher-order effects, like its neutrino-absorption cross section, to calculate some ...

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The expansion of the universe is related to its cooling. Temperature is defined as the inverse of the partial derivative of entropy with respect to energy: $$\frac{1}{T}=\frac{\partial S}{\partial E}$$ The derivative is taken at constant volume. This is the definition of temperature. This definition only makes sense for macroscopic bodies. I.e., ...

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I posted more-or-less the same thing in response to another query about whether "tired light" still holds any water. I don't think there is any doubt that the redshifts we see in the spectra of distant objects are real doppler shifts that can be explained by an expanding universe. A crucial piece of evidence that seems to be ignored by almost everyone ...

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The big bang may have two different meanings: First, a phase of very strong inflation of the early universe. In this sense the big bang is widely acknowledged. But we have very little ideas about what was happening before this inflation phase as it seems that the known laws of physics cannot be applied. There are two main possibilities: Initially the ...

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There is difference between what there is and what we observe. We do observe finite amount of matter and energy exactly because it all started somewhere and moves away, while speed of light is pretty constant. In that, most useful, sense universe is finite. We can speculate about size of what "there is", but it still will be baseless theories.

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I'm not dealing with most of this ; the reason most physicists are convinced that something like the big bang model explains many aspects of the universe is that the model has been verified against many observational tests. These are not just limited to the expansion of the universe and predicted properties of the microwave background. In answer to a couple ...

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Consider the function $e^x$: it is monotonically increasing and yet defined for all negative $x$. Just because something increases monotonically doesn't mean it must reach infinity (or even its maximum value) in a finite amount of time. As a side note, please don't refer to entropy as disorder. It's very common but also very wrong: ...

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... drop to zero and flip the sign, significantly before the scale reaches zero (i.e. before Big Bang)? That would mean not more and not less than that there was no Big Bang singularity. The universe could have started with a volume which was small but not zero. As we are not able yet to apply physical laws to the very first period of the universe, ...

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In classical thermodynamics, only changes in entropy ever matter ($dS = \dfrac{dQ}{T}$ for reversible processes), so it is not meaningful (though it may be convenient) to define an absolute entropy. HOWEVER, in statistical mechanics, entropy has a probabilistic interpretation: $S = -k_B\sum_i p_i ln p_i$, where $k_B$ is Boltzmann's constant and $p_i$ is ...

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It seems that the answer to my question is that there's a difference between the pre-inflation universe "singularity" and that of other types of singularities like a black hole. It wasn't a singularity as describes a black hole, it was a point in time where the scale of the universe was zero. All that exists didn't occupy a single point in space in the way ...

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I'm not a quantum cosmologist, but I am an early-universe cosmologist, so I can give you my opinion after having read this paper. The article claims that Bohmian trajectories is a valid replacement for geodesics. This was claimed in the very beginning of the paper and not much is offered in the way of defense for this assumption. That's not to say that it's ...

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The "explosion" wasn't caused by matter being under high pressure. It is thought that during the first few moments after the creation of the Universe, it went through a phase called "inflation", where there was no matter or radiation or anything else, only potential energy, which lasted $\sim10^{-34}$–$10^{-32}$ seconds and blew space up by a factor of at ...

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There are a number of models for the universe over the years. The Big Bang as you show it in the figure has become the "standard model" for the creation of the observed universe as we know it because it fits observations, i.e. data, using known theories and behaviors from elementary particle theories. This model has been evolving as data are added in our ...

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While this work certainly investigates an interesting point, I think simply replacing geodesics in GR with similarly looking quantum trajectories does not solve the issues here. Finding the Friedmann equations while assuming large-scale homogeneity and isotropy is no surprise to me. There are a number of people working on so-called Big-Bounce Cosmologies. ...

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Your question is: why is said that the space is expanding ? The answer that everyone are aware, by consensus : We see a reddening of the light received from distant stars and galaxies for the same stellar processes we see in the sun and nearby stars. One way to interpret it, the official way, the only one that your teacher and scientists do know, is: the ...

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"The way I see it, space, true "space" is literally nothing" Most physicists and I believe many philosophers would disagree. The notion of "Nothing" is impenetrable logically: "nothing" has neither any properties nor relationships with anything else that can be reasoned about. The best you could do would be to assert that "nothing" is somewhat like the ...

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According to inflation, strictly speaking, space is still being created. I find this idea very interesting. Also, the idea of space as being an empty void to be filled is out of date. Space is full of fields, even when there are no particles present.

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If you had empty space and then matter expanded from a point in it, then some of the matter would be in the center, seeing everything moving away from it. Some of it would be near the edge and see darkness filling half their world. We see matter moving away, and it seems unlikely that we just happen to be so close to a center of the universe. So we look ...

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I think there are two key misconception you seem to be having here. Before I say what it is, a disclaimer: you haven't filled in your profile, so I have no idea of your level of knowledge. Therefore, if my answer seems to be telling you how to suck eggs, please be aware that I am not meaning to be condescending. The misconceptions: 1) You seem to be ...

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The expansion of the universe referred to in the big bang theory is not about objects moving apart through space, but about space itself expanding between the objects. So the short answer is that the reason some objects are so far away now, is that at some point in the early universe space was expanding faster than light could move through it, effectively ...

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This is a typical conundrum created in using classical thinking in combination with some relativistic concepts (e.g. photons experience no time) Infinite time dilation (i.e. no time elapsed for the photons) is immediately related to infinite length contraction (there is no distance to travel) and the only conclusion here should be: the frame of photons is ...

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