Space expansion Definition and 40 Threads

Being material observers, we do not expand with the universe. Our ruler for measuring its increasing size does not expand either - its scale does not change. If I identify the ruler with a metric, then from my perspective it should be invariant both spatially and temporally. If it expanded with...

I'm studying the nonrelativistic-matter perturbations if the expansion of the Universe is driven by a combination of components. I'm currently Following this document (The growth of density perturbations) from Caltech. However, the author doesn't explain how he has found the solutions for the...

Are there any kind of observed and experimentally verified processes or mechanisms where photon emission occurs and which are directly cause by spacetime expansion in some way?

Prof Wetterich has proposed that atoms are shrinking rather than the Universe is expanding. Here is a 2013 Nature News article describing his theory: https://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379 Here is his 2013 paper "A Universe without expansion"...

I was wondering if it would be possible to see the history of space if one was to stand at the edge of the universe as it expanded faster than the speed of light. If the universe was to be expanding faster than the speed of light right now, and I was able to somehow go there and keep up with it...

Imagine a Universe where the Hubble parameter is truly a constant, in both space and time. How much smaller would such a Universe be 14 billion years ago compared to today? Using the Hubble parameter in terms of scale factor: ##H(t) = \frac{\dot{a}}{a}## leads to the differential equation...

What do people think of Fulvio Melia's argument for the necessity of "zero active mass" in FRW cosmologies? (i.e. the overall equation of state must be ##\rho+3p=0## at all times) Here is a link to an interesting lecture video: Here is a recent paper: https://arxiv.org/abs/1807.07587

1) space is expanding at an increasing rate, therefore things are getting farther from each other and therefore increasing in velocity. 2) the faster an object moves relative to another, the more mass it has 3) supermassive objects can turn into neutron stars, black holes, etc. Therefore, will...

I am looking at a couple of very interesting papers, published in MNRAS, that deduce, that the accelerated expansion of the Universe we observe can be attributed to gravitational waves, produced by a very distant merger of two or more universe-mass-scale black holes. The last one is on the...

If the uncertainty in the age of the Universe is ##\Delta t## then the Uncertainty Principle implies that it has an uncertainty in its energy ##\Delta E## given by $$\Delta E \ \Delta t \sim h.\tag{1}$$ If this energy fluctuation excites the zero-point electromagnetic field of the vacuum then a...

Dear PF Forum, I've been wondering about how on Earth (and I do mean it, from earth :smile:) that we know there's a galaxy 20 billions light away. Considering that https://en.wikipedia.org/wiki/Age_of_the_universe is 13.8 billions years old. But before I'm asking about Supernova Ia and Hubble...

Is it possible that the increased acceleration of the universe could be caused by the cancellation of gravity waves? If not, what would be the impact on space-time when gravity waves do cancel out?

This had me thinking for a while. Imagine a photon emitted by a very distant object at a redshift of z = 2.0 for example. As the photon travels through space, due to space expansion the photon's wavelength will shift towards red. With an increase in the wavelength there must come a decrease in...

Hi, Apart from the redshift of electromagnetic waves (which we do observe), is there any other evidence of space expansion that is not based on redshift? If not, is there a theoretical experiment that could let us observe and measure space expansion? Here I am assuming that space is not just...

Good day. I do not know much about cosmology, rather computer science, but the following theoretical question bothers me a little. Some scientists, like Tegmark, Wolfram, Zuse or Fredkin, support the idea that the Universe might be just computation. Computable means that something can be...

From what I have read, it sounds like GR predicts that under certain conditions, space itself will expand. If this is incorrect, please just let me know and ignore the below. While I don’t know the math involved in GR, I can understand explanations of how gravity distorts space-time, to alter...

Time can have 2 different connotations. One is inherent to an entity in spacetime by itself, say a lonely radioactive atom. It may decay or it may not decay, independently of what's going on around it. But another involves interactions between particles, for example "it takes 60 seconds for...

If spacetime is a single entity, shouldn't it be simply logical that space expands? We know that spacetime is one single entity, there is no space AND time, but just spacetime. As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the...

Dear PF Forum, Thanks readers for giving me so many explanations about space expansion in my previous thread. However, I've forgot to ask 2 things that I'd like to know the answer. Space is expanding. According to Hubble Law? What cause space expands? 1. Dark Energy? 2. Does space its own have...

Do models of the expansion of space-time manifest that expansion by an increase in Plank-length, or by additional Plank-lengths appearing in between existing particles? I think it must be the latter because the Plank-length is not an empirically measured value and the constants it derives from...

So what I don't get about the expansion of space, and what I'm assuming one of you can explain to me, is that it seems like if space were expanding, how would we have any way of noticing it? It seems like as the distances between particles expands, so must the size of the particles themselves...

Hello everyone, I have a question about metric expansion of space. According to Wikipedia (ok probably is not the best source, but I have only a qualitative understanding of physics) the expansion occurs only at scales larger than galaxy clusters...

If gravity waves are supposed to propagate at c, how could the theoretical warp drive wave propagate at greater than c values? I have heard and red and understand that space itself can expand (and warp? ) faster than c and, in fact, that will happen in the future when space in between...

As I understand, every point in the the universe may be considered for that observer to be the center. Everything seems to be expanding away from that point. From our perspective on earth, the further away we observe, the faster that space is expanding, up to, and faster than the speed of light...

A few years ago I came to this forum to ask about the issue of space expansion...and the answer I got then was that the expansion is the nature of the space (or space-time) according to relativity or some other more advanced theory...no one used dark energy to explain the issue at that...

In a counterpart of General Relatlvity called Field Theory of Gravitation where spacetime is really flat and spin-2 fields cause gravity. See: http://arxiv.org/pdf/gr-qc/9912003v1.pdf Space is already there as the following description state: "Cosmology is another field of application...

I devised this using a setup that I found easiest to imagine, while trying to be clear on how modern physics considers space to expand. We have a nifty device that can cause spatial distortion (no, you cannot has one!). Before we use it, we lay down two meter rulers, end to end. They both...

Hi! I've been reading these forums off and on for a year or so, and finally registered! There have been other threads here on space expansion vs. time speeding up, but I have some questions that weren't addressed. Background: I was told by a notable astrophysicist, Neil DeGrasse Tyson...

Greetings, When a photon travels through expanding space, it's frequency decreases and it has less energy. The energy of photons of the cosmic background radiation have decreased dramatically. Where does the energy go? Is it lost? Thanks

What caused and is causing space to increase or expand? Greetings, The answer to a major question has raised another problem...then solution...then questions. Why didn't gravity keep everything together at the big bang like a black hole? Because *all* of space was filled with...

Space is expanding. This is evident from the redshift of distant objects. My question is what effect does it have on smaller scales? Since the expansion of space carries objects along with it (necessary for the objects to be farther) wouldn't the expansion within galaxies slowly pull them...

So if space is expanding everywhere, does this mean the space in my room is expanding? I'm assuming that its expanding so slow because of the small scale, but is there still a measurable amount of space expansion occurring in my room in any time interval? If i were to wait a few billion...

As a steady stream of light particles (or any other form of measurable energy) from some distant star, say a billion light years away, makes its way toward us, shouldn’t space expansion cause consistent and measurable gaps to form between each light particle, causing a blinking effect? The...

It is my understanding that with the passing of time, space actually expands in size. In about 14G years, space will be twice its current size. What will this do to black holes (BH)? If a BH is in intergalactic space and has nothing to consume, if space actually expands, the density of the BH...

Confused about space expansion?? Me too. Space expansion is occurring from what we can observe. In another thread about gravity i read that gravity is space time essentially, as if you introduced an object into Earth's atmosphere it would instantly be subjected to Earth's gravity as it...

Does space expansion theory obey relativity laws?? I've been wondering about the idea of space expansion that's been used to explain red shift effects in most of our universe, which forgive me if i am wrong is nothing(space) propelling matter. Please could someone tell me if i am...

Just a fast question, Ill be glad if you answer it. I was wondering how fast does the space expand, but I wasn't able to find answer anywhere around the internet, the only thing I was able to get is the rate of expansion - 72 km/sec/Megaparsec, but I don't know if I can derive the speed of...

I don't understand what you mean with: universe is expanding. Is space that -as an elastic membrane- is expanding or galaxies have a proper velocity? And however, shouldn't all become bigger during the expansion? for example, in a sphere-universe of diameter 100 metres there is a...

[SOLVED] Isn't space expansion expectable? Since Einstein showed that space and time are in fact entwined as a single entity spacetime, it makes me wonder if it's not just natural that we see space expanding with time. As we examine spacetime forward in time (looking at regions where "the...

General Relativity GR, and Loop Quantum Gravity LQG as well, says that space is derived from the gravitational field of matter. But the total matter in the universe is constant; whereas the space of and the amount of Dark Energy in the universe is growing. It is supposed that most of this...