Astronomers posit that we live in an area with below average density
phys.org
phys.org
https://arxiv.org/pdf/2311.00215.pdf
MOND hasn’t been having the rosiest of time lately. https://academic.oup.com/mnras/article/527/3/4573/7342478?lo...
These results seemingly worsen the recently established tension between the inferred value of H0 from early and late times Universe probes, which has been argued to potentially be the sign of new cosmological physics (see for example Refs. [78–83]). This might appear to be in contradiction with the possibility, explored for example in Refs. [74, 84– 93], that local gravitational physics could alleviate the Hubble tension. Amongst these, a class of models achieve a lowering of H0 under the assumption that we live in an underdense region, whose inner expansion rate is on average larger than the background one. Some results in the literature, see for example refs. [94–96], seem to corroborate the latter assumption finding evidence of local voids which averaged on spheres of r ≳ 100Mpc have density contrasts of δ ≤ −0.1, unexpected within the ΛCDM model. Computing the average density contrast of a sphere centered in Laniakea with radius r ≈ 110 Mpc (i.e. the average distance of the boundary of the ellipsoid from the center) using the CF4 reconstruction we found δ ∼ −0.06, within the prediction of the concordance model (see for example Fig. 6 of Ref. [55]). However, this sphere is not centered in the Milky Way, which might explain why the result differs from the aforementioned ones. Indeed, overdensities such as Laniakea are surrounded by voids (from which they have collected matter), and therefore any sufficiently spherical average will include these under-dense regions. On the other hand, Refs. [55, 79, 97] also found no evidence of any large void or overdensity, thus disfavoring a local resolution of the Hubble tension. Our analysis corroborates these results, suggesting instead that the tension is likely to be (slightly) worsened by Laniakea’s backreaction. An important caveat, however, is that our analysis does not exclude the possibility that large voids in the annular region between 110 − 400 Mpc outside Laniakea could balance and overcome the backreaction from Laniakea, like a rather picturesque Matryoshka doll. Alternative modelling choices accounting for the impact of these voids are therefore required to fully understand the impact our cosmic environment’s gravitational backreaction, which will be the focus of forthcoming studies.
The picture I've been given is living on the surface of a balloon. Draw a few dots on the surface, blow in the balloon. Every points expands away from every other point.
Of course with a balloon there is always an outside, but the point is that it isn't expanding like if it were pushing against an external wall.
It's expanding everywhere. It's expanding in the space between your fingers. If you look there, it's not pushing against any outside. There is just more space per space, the longer you let it age.
That's not the case (at least not yet [0].) Anything that's bound together by local forces such as gravity or electromagnetism isn't affected by the universe's expansion. You have to get to the scale of the space between galaxies (and even then, galaxies which aren't orbiting/colliding with one another) before the expansion has any effect.
[0] One of the hypotheses (of many: https://en.wikipedia.org/wiki/Future_of_an_expanding_univers...) of the future of the expanding universe predicts that the expansion rate will increase indefinitely, do the point where the expansion energy (dark energy) actually does become enough to expand the space between stars, planets, and even matter (and eventually the atoms in your body), but we don't really have any proof of this. This theory is known as the "big rip": https://en.wikipedia.org/wiki/Big_Rip
But the point is the space between your fingers isn’t literally expanding. It’s not like the universe is just expanding everywhere uniformly, which is what tux3 seemed to imply. The distance between your fingers is staying the same, and so is everything else nearby.
I thought this is exactly what most modern physicists believe.
The wavelength of photons travelling in space is supposed to scale perfectly with the expansion of the universe, like an ink dot on a balloon getting wider as the balloon is being inflated.
This is why we have the Cosmic Microwave Background consisting of photons that would have been maybe in the UV range when emitted by excited atoms back when the universe became "transparent".
“Space” is expanding uniformly, yes. But not the matter that exists in space, nor is the distance between local matter increasing.
It’s not like the entire universe is undergoing a magnification of scale where everything is enlarging including each individual atom.
So it’s misleading to say “the space between your fingers is expanding”, because it might give the impression that the distance between your fingers will actually be larger as time passes. This is not the case.
If “space is expanding”, but the matter inside this space is unaffected by said expansion, then you’re left with a very subtle phenomena which is easy to misunderstand.
I struggle to wrap my mind around what that means for concepts I learned long ago in college physics... what is an inertial frame of reference then? What about the speed of light and relativistic effects?
I suppose the answer is back to the original statement that the effect is really only noticable at cosmological scales. Locally, it's well below any margin of error?
True. I think that the best way to explain this to someone who does not know about the phenomenon is to start with the phrase "existing space continuously creates new space" and everything should follow from this fact.
This is roughly similar to how planets don’t collapse due to gravity, because the stronger electromagnetic repulsion makes them stable bodies, but this doesn’t mean that the gravitational field still isn’t there throughout the planet.
Otherwise we increasingly approximate a transition to "nothing" into infinity.
Change 'where' to 'whether' and everything will fall into place.
I've wondered for many years if our universe isn't like a supersaturated (SS) solution (1), where mass/energy and the fabric of space-time itself can eventually combine and settle into a balanced but very precarious state, like an SS solution comprised of Mass-Energy-Space-Time [MEST] as a single "fabric" (which perhaps was the state of the universe prior to the big bang, and will be again after the universe' heat death).
Say this large swath of peaceful-yet-highly unstable MEST fabric is perturbed slightly (like dropping a seed crystal in a SS solution) - then matter, energy space and time precipitate outward in a rapidly evolving chain reaction. The horizon of this chain reaction would be a place where matter and energy particles are continually being generated (liberated) from the serene METS fabric, which generates a gravitational pull in all directions.
So, all of the mass in our universe is actually being pulled outward, and because this precipitation horizon is expanding (growing in surface area), it generates an increasing amount of matter and hence an increasing amount of gravity that ultimately pulls matter within this horizon outward - at an accelerated rate. Perhaps the big bang was really, instead, a 'big fizz' - chain reaction of precipitated matter and energy from an initial supersaturated MEST fabric.
Or maybe I'm just full of silly nonsense. Either way, would love to hear an actual astrophysicist 's take on that idea. Maybe gently tear it to shreds:)
The degenerate matter state is like a partial order of time. https://en.wikipedia.org/wiki/Causal_sets
It is a purely speculative article.
https://www.youtube.com/watch?v=-kTe0xRAU1w
Seems more concerns about the issue of cosmological constant and its disagreement with observation and current model.
I have a theory in my head for years now. It is probably wrong, but here it comes:
In an infinite universe the total amount of gravity that affect us in one point in space is defined by the event horizon if we assume that gravity travels with light speed. Every atom in the universe has a very small influence on us. But this event horizon expands with light speed all the time. I wonder if this could lead to very small but permanent increasing gravitational pull from all directions at once. In other words, and increasing inflation.
In our actual model of the universe, using the correct theory of gravity, the "total amount of gravity" affecting us (or any point) from the rest of the universe (i.e., once we factor out local influences like our solar system and our galaxy) is zero. That is because the average matter distribution in the universe is the same in all directions from us, so the "gravity" from it cancels out. The average matter distribution in the universe affects its overall rate of expansion over time, but this is not the same as the kind of "gravity" you are thinking of.
Also, while our universe does have a cosmological horizon (due to accelerating expansion), this horizon does not work the way your hypothetical "event horizon" does.
In short, your "theory" is not even wrong, because it doesn't even start from a correct underlying theory of gravity.
I think this statement is really the crux of the counter-argument. Your statement that matter has average uniform density in all directions around us is obviously only correct in sufficiently large frames of reference; there are galactic voids, and galactic-super-strands, uniformity really only exists within the "mathematically and hypothetically infinite" frame.
You should expand on why the cosmological horizon does not function in the same way the GP's "event horizon" analogue does; and/or possibly, expand on how large the frame would have to be to achieve reasonable uniformity.
Yes, agreed. But the distance to our cosmological horizon is large enough that the averaging assumption is fine on that scale.
> You should expand on why the cosmological horizon does not function in the same way the GP's "event horizon" analogue does
Fair enough. But first I'll reference an excellent paper by Davis & Lineweaver (2003) [1]. You can find much more detail there than I'm going to give here.
Some simple facts about our cosmological horizon are:
(1) It is currently receding from us very slowly (in terms of proper distance); and asymptotically it will be at a constant distance from us forever (in more technical language, our universe approaches de Sitter spacetime asymptotically, as all other matter and energy becomes negligible compared to dark energy; and in de Sitter spacetime the cosmological horizon is at a constant distance forever). It certainly is not receding from us at the speed of light.
(It is true that the horizon is a lightlike surface--but that does not mean it's receding from us at the speed of light. In fact, counterintuitively, to the extent it is "moving", locally, in any direction, that direction is towards us, not away from us! But due to the curvature of spacetime, its proper distance from us will asymptotically be constant.)
(2) It is not a boundary between things that can affect us gravitationally and things that can't. To the extent there is such a boundary in GR, it's our past light cone (which is shown in the diagrams in Figure 1 of the paper I referenced). But one also has to consider the other caveats I gave in my earlier post.
(3) The cosmological horizon is a boundary in spacetime, not space. It is a boundary between the region of spacetime that will be able to causally affect us into the infinite future, and the region that won't. Particular objects, as they recede from us, will move beyond the cosmological horizon and will then no longer be able to causally affect us. But events that occurred in those objects before they moved beyond the horizon will still be able to causally affect us--though of course it will take time for those effects to propagate to us. For example, the light we see from distant objects is a causal effect, but we see the objects not as they are "now" but as they were when the light was emitted. It's quite possible for a distant galaxy whose light we are seeing now to be beyond our event horizon "now", but they weren't when they emitted the light.
The spacetime diagrams in Figure 1 of the paper I referenced can be very helpful in making all this clearer.
> expand on how large the frame would have to be to achieve reasonable uniformity.
A few billion light years is certainly large enough. Our cosmological horizon is quite a bit further away than that (about 16 billion light years, according to the paper I referenced).
Can't gravity have an infinitesimally small effect, which means this matter distribution has to be perfectly balanced for its gravity to cancel out to zero at some point?
In principle, there are of course effects due to the matter distribution not being perfectly spherically symmetric about us. But except for the obvious effects that are due to nearby obvious objects--like the Earth, the Sun, our galaxy--in practice the gravitational effects of the rest of the universe on us are negligible.
Only in speculations that we have no way to test now or in the foreseeable future. We are many, many orders of magnitude away from being able to probe spacetime on such scales.
As I have pointed out upthread, the GP's "theory" is not correct, so it doesn't account for anything.
"Expansion of space" is just a consequence of the overall spacetime geometry of the universe, which is due to its overall average matter distribution (and to dark energy, which is what is causing the expansion to accelerate).
What would result in a contracting universe is a large enough density of matter (about a factor of 20 larger than the actual density in our universe if we just look at ordinary visible matter). But this does not mean "increasing gravitational force". As I have pointed out in other posts upthread, the "gravitational force" on a given piece of matter due to the rest of the matter in the universe (if we leave out local influences, like our solar system or galaxy for us here on Earth) is zero. This is true regardless of the current state of expansion or contraction.
Does that suggest that our awareness of the galactic bodies itself is somehow influencing them, as in the double-slit experiment?
In the double slit experiment, the awareness is not influencing the outcome. The act of measuring is. Pretty sure the act of measuring the galactic bodies has no impact on them in any meaningful way.
As I understand the double slit experiment, this is a fundamental property of light as a photon exhibits wave-particle duality. If so, retrocausality in this case would just mean the fundamental wave function can be collapsed into actuality without time or space being involved.
Am I far off base?
There is no way to verify that. Big parts of quantum physics is more or less pseudoscience.
[citation needed]
So they could capture the light and attempt to capture the "same" light again later to try to verify or change the result they received.
I did some more reading about it after posting, and the original experiment was intended to test if light "chooses" to be a wave OR a particle in a way they could affect, and that it could only be one or the other at a time. The truth seems to be more that it acts as both at the same time and whatever sensor equipment you use to pick it up is what it acts like.
The issue is that not all galaxies are moving away from us. The ones that are closer to us have a lot of peculiar velocity [1]. This means they can be moving toward us or moving tangentially to us or any other direction. If we want to characterize the expansion of the universe as a whole, we need to account for this in our models. It turns out to be a lot more complicated than we previously thought.
The crisis in cosmology (aka the Hubble tension [2]) is that our two means of characterizing the expansion of the universe, models of the cosmic microwave background (CMB) and measurements on the cosmic distance ladder using standard candles (Cepheid variables [3] for up-close measurements, Type Ia supernovae [4] for more distant measurements) disagree with one another, and that disagreement is getting worse, not converging.
[1] https://en.wikipedia.org/wiki/Peculiar_velocity#Cosmology
[2] https://en.wikipedia.org/wiki/Hubble%27s_law#Hubble_tension
"Awareness" is not a thing, not even in the double slit experiment. The term 'measurement' refers to a specific kind of interaction that bridges quantum systems with classical systems, although I believe a good case could be made that these waveforms never actually fully collapse.
Likewise, if there is anything special about Earth's position in the greater cosmos, it would be a trick of perspective or perception - unless there are any completely disruptive new discoveries about the nature of reality. However, my money would be on the fact that the universe is simply not as uniform as we thought.
As an analogy, consider the 2 dimensional surface on the surface of a balloon. As you inflate the balloon, the distance between any 2 points increases, and it increases more the farther away the points are from each other.
It wouldn't - but it may influence how we measure distance. If we're using the wrong distance measurements then we're calculating the speed of expansion incorrectly.
My intuition of 'things far away moving away faster' causes me to conclude that things far away are therefore accelerating, because the more they move away the faster they move away, but my intuition is probably wrong as it doesn't line up with the raisin bread analogy.
This place isn't debate club and nobody's getting a prize for winning an Internet argument.
Theoretically a space/" " is just another character, so maybe. But practically speaking, it would probably be legally filed as "space", and the company's trademark would be on either them naming their company "space", or on them stylizing it as a " ". That's my best guess, at least.
Am I right?