Universe Definition and 1000 Threads

I have come across a questions which reads "Why does even photon travel?". After reading special and general theory of relativity, this one bugs me all over. For a particle moving closer to speed of light, clock slows down and the space around it contracts. I see a photon travel and I see things...

Why is the accelerating expansion of the universe not explained simply by the perspective of acceleration caused by a constant (non accelerating) universal expansion? Let's take two particles (X,Y) in a 1d expanding universe where it expands universally by 1 unit per per unit time. O represents...

First we have to agree on the definition of infinite: https://en.m.wikipedia.org/wiki/Actual_infinity So: - Potentially infinite is the process of continued and potentially endless iteration (IE a limit). - Actually Infinite is the result of an unbounded number of iterations; IE NOT DEFINED...

I have an image of the block universe that is probably similar to how many people view it, as a 4D non dynamical construct where the "present" is a slice of this block and this slice moves forward from the past to the future. Furthermore, depending on your relative velocity, your slice may be...

Can't find a program at my level to simulate orbital or electron motor velocity, so is it possible to use universe sandbox2 to implement the atomic world to confirm the orbit? It is difficult to convert static electricity into gravity... It's hard to tell what the distance is between an electron...

After reading some classical physics I learned that the entire physics is actually studying the combination of 3 quantities mass, length and time. Anything in the universe is just the combination of mass and length and time. And why i say that? Because if you notice the formulas that the...

I read in a so called sci-fi website from my country that as the universe is expanding we may not be able to get to certain galaxies in a possible interstellar travel. That information seems ok for me. But then there was a comment by a reader concluding from the website post that there is coming...

I have just simple Knowledge, but recently a question came To my mind. An Entity Living in 2d Space cannot Directly Observe a sphere, but rather a Circle Changing its Size when a sphere is traversing trough the observable 2d plane. Similar, a 4d sphere would Appear To us as 3d sphere Changing...

Just a thought I had today. I read an article that mentioned how the universe is expanding faster and faster and how dark energy may be the cause for this, but I wondered, what if the accelerating expansion of the universe is not caused by dark energy but simply by the gravitational attraction...

I know nothing about physics, but it would be great to get an answer to my questions of anyone can help: 1) If everything that exists does so within the universe, does it make any sense to say that the universe 'grows'? Against which yardstick is any growth being measured? 2) Would the energy...

I have a hypothetical universe where the distance between two points in spacetime is defined as: $$ds^2 =−(\phi^2 t^2)dt^2+dx^2+dy^2+dz^2$$Where ##\phi## has units of ##km s^{-2}##. The space in this universe grows quadratically with time (and, as I understand it, probably isn’t Minkowski...

One idea I have been hearing about recently is the Janus universe https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.113.181101 Im hoping someone might have a go at giving a layperson explanation of this paper. As I understand they are saying a system will eventually come to a...

It is sometimes said that entropy is "unlikely" to return to the "pattern" that it came from, for instance: if we have a vat with blue gasmolecules and white gasmolecules separated by a slit, if we remove the slit, the blue and white molecules will mingle, unlikely to return to their separated...

Hello. I'm not entirely sure how to go about asking these questions - so please bear with me if I trip up in the framing of them. Any helpful correction or advice is welcome, btw. I've done some reading up on how inflationary theory tries to solve the horizon problem...

We have set the Earths' Voltage at zero volts but is this 'earth voltage' applicable throughout the Universe. If not, is it relative in the same manner as spacetime and is it calculatable?

We keep hearing that "It all started with the Big Bang" and how "Everything was compressed into an infinitely small dot" and suddenly it expanded. Personally, I've wondered if this is more of a misinterpretation of distance, just as the headlights of heavy traffic on the highway looks like a...

Is it possible that our 3D universe is just a shadow in 3D space of 4D or higher dimensional reality? And we are limited in our perception of it by our 3-dimensionality. In other words like Flatlanders would just see s square of a cube sitting on the plane or passing through the plane...

We have a Universe that can be seen by Hubble maximum and we can imagine millions time more than that but if there is no boundary, there may be another millions of universe. Suppose we gather this all millions of universe and say this is a giant universe of universes and thus matter do not stop...

How does the expansion of space cause light to lose energy?

This idea has always bugged me: If we are looking at the past when observing the redshift of far away galaxies (ex: 10 billions light years).. then how can we tell that the universe at the present time is still expanding at same rate, deaccelerated, or stopped expanding?

Does the relative density of the early universe contribute to the red-shift of distant galaxies? If so, by how much? How would this be calculated? Asked another way : Assuming both the early universe and the current universe are flat, could the relative difference of their space time metric...

This is a question I was looking at based on Relativity and John Wheeler's one-electron universe theory. https://en.wikipedia.org/wiki/One-electron_universe My question is this. The faster you move towards the speed of light, wouldn't everything in the universe contract to a single particle...

I assume that before the Big Bang, there was no Higgs Field, since there was no universe for it to fill. I assume that at the moment of the Big Bang, it began to seep into every corner of the expanding universe and was carried by inflation, that is, moving faster than the speed of light by the...

If you guys haven’t noticed through my posts so far is that I am interested in the Big Bang. I am considering the concept of the Planck length, but I may have some confusion. How did Space time start? Did the universe start from nothing and created units of spacetime? Could the original size...

What are the effects considered by the presence of sterile neutrinos and the early universe, and the neutrino decoupling era?

It’s confusing how a universe can have finite mass but infinite size. Could the universe be like a tesselation, where you always end up where you started? Not like a closed universe, but where space is just copied over and over?

Hello, this idea of holographic universe is mind boggling to me. If we are 2d and everything we see is actully 2d hologram, like picture on the monitor https://metro.co.uk/2017/01/30/our-entire-universe-is-an-illusion-and-reality-is-actually-a-2d-hologram-say-scientists-6415724/. There is...

How large was the universe at its moment of creation? Do we have a real answer for this? If so, how was this found out?

I have read that the early universe had a very low entropy. I don't understand why. A giant ball of plasma at billions of degrees K with particles moving in all directions. It seems like the definition of total disorder. Why is the entropy considered low?

Ok, so just a quick question, first law of thermodynamics basically states that matter cannot be created nor destroyed aka it can only change states and turn into energy or vice versa, the second law states that, IIRC, for perfect isolated systems entropy remains constant but for our universe...

Hi, I was watching a documentary by Professor Brian Cox and he said human life is only possible for 10-86% of the life of the universe. He based this on the fact that the age of starlight we are currently in is a very small event in the life span of the universe and human life obviously can’t...

I was just curious because if space can expand faster than light, doesn't that mean there will be a lot of space that we just can't see? Do objects just vanish because we can't see them? For instance, if a hypothetical alien lived in MACS0647-JD galaxy which is 13.3 billion light years away...

This thread is a split-off of this post: https://www.physicsforums.com/threads/do-macro-objects-get-entangled.946927/page-2#post-5997089 So my issue is this: if, for convenience, we use a Copenhagen interpretation, and we measure an observable WF ##\alpha |A \rangle + \beta |B \rangle##, then...

IMO, it is very anthropocentric to specify that the Universe is 13.7 Billions years old, i.e., using Earth's period of revolution around a minor sun as a baseline. There might exist a Galactic timeframe that would/could suggest that we actually live in a very "young" Universe. Just for fun, I...

Hi Forum, I've been pondering about this for a while and I hope someone here can help! Imagine two bodies orbiting each other in tidal locking (https://en.wikipedia.org/wiki/Tidal_locking) just like Pluto and Charon, or as Earth and a geostationary satellite. Nothing else exists in this...

Homework Statement Consider the Godel Metric in spherical coordinates as on page 6 here; ds^2=4a^2\left[-dt^2+dr^2+dz^2-(\sinh^{4}(r)-\sinh^{2}(r))d\phi^2+2\sqrt{2}\sinh^{2}(r)dt d\phi)\right] This is a solution to Einstein's Equations if we have ##a=\frac{1}{2\sqrt{2\pi\rho}}## and ##\Lambda...

I read somewhere that it is not possible to prove that Our universe (from big bang to now) have different principles that inside of black hole. I think that one physicist said something like this. Or in different words: are principles of our universe different as inside of a black hole?

Hello everyone, I don't normally come to Astro/Cosmo forums, but I stumbled upon a discussion between a mentor and a PF member here, which involved explanations on the geometry of the universe: difference between boundless vs unlimited, 2-torus vs 3-torus, why are tori boundless, etc. This got...

According to Source: They discover a mega cluster of 14 galaxies that originated only 1400 years after the Big Bang This is the most active place in this universe

Hello.Long time,No See. I am just wondering if anyone can point be in the correct direction. Preferably with a link or a book that I should read as i would like to understand, not just know the answer. I am just wondering how much the number of stars in the universe has changed over time...

A black dwarf is the dead ember of what once was an average star which expired as a white dwarf. Is it possible that these may account for at least some of the dark matter?

Greg Bernhardt submitted a new PF Insights post Lessons From the Bizzaro Universe Continue reading the Original PF Insights Post.

What is the deviation in the expansion of the universe exactly quantified, when I would assume general relativity and project it backwards? As a statistician I am asking for data, for either the backwards projected general relativity case and either the real expansion case, as it is...

I know this is getting into the highly speculative, but given the mathematics that has been done on versions of these concepts that are not known to be inconsistent: In the inflation concept which leads to a multiverse of the "bubble universes" type, in which the laws governing the space-time...

I don't understand why CP violation is insufficient to explain the observed baryon asymmetry? in every article I find it says: "we know its insufficient..." without an explanation. I will be glad for an explanation and for articles that deal with this issue. And how do you solve it using the...

Conceptually, at least, this is a simple question, although I recognize that it might be hard to calculate in practice from available data. The matter-energy budget of the universe is measured (in a model dependent way) to consist of a certain percentage of dark energy, a certain percentage of...

My somewhat ropey understanding of entropy is that it is a measure of order/disorder and that in a closed system entropy always increases. Was discussing it with my teenage daughter. Whilst trying to convey my limited understanding it struck me that if the universe is contracting (we had also...

General Relativity equations tells us that the earliest time of the universe which our physics can tell us had infinite space and infinite density (i.e. matter). Then space started expanding, thus increasing the distance of any 2 points of that infinite dense matter, thus making it less dense...

<< Mentor Note -- after a very long Mentor discussion, we acknowledge that this paper, while potentially controversial, has been published in a reputable peer-reviewed journal. We believe that a discussion of this paper can be useful and constructive. Thanks >> This paper of mine was...

I've read that until 5 billion years ago the rate of expansion of space was decreasing. How do we know that it was slowing down? Does the uniformity of the CMB mean that the rate of expansion is identical throughout the entire universe - or does it just mean that the average rate of expansion...