Dark Energy May Not Exist: Something Stranger Might Explain the Universe

I don't see where this explanation is strange, it's straightforward GR (General Relativity), as much as one can call GR "straightforward." It's a simpler explanation than inventing Dark Energy, for which we have no direct observational evidence.

This is referring to timescape cosmology and the recent paper (Supernovae Evidence for Foundational Change to Cosmological Models) that was discussed 11 days ago on HN:

https://news.ycombinator.com/item?id=42495703

I hope this is true, it seems to make sense even to someone with absolutely no knowledge of this stuff, like me. It seems simple. I like simple.

Just a thought – could the existence of something like dark matter actually be causing the differences in the rate of time in different parts of the universe? If dark matter affects gravity in ways we don't fully understand, maybe it's also influencing time dilation across the cosmic web. That might reconcile these two ideas – with dark matter being the underlying cause of the time variations that timescape cosmology talks about. I'm curious if anyone has explored this connection or if it's completely off base?

Come to think of it, another angle to consider – could it work the other way around? Instead of dark matter causing time dilation, could time dilation itself be a root cause that leads to the accumulation or creation of something like dark matter? If time moves differently in certain regions, maybe it affects how matter and energy interact over cosmic timescales, creating the conditions we interpret as dark matter. Just wondering if this has been explored or if it’s way out there as an idea? I’m definitely not a cosmologist!

Anyone happen to know what the time dilation factor would be for the Milky Way at somewhere near the outside edge and somewhere nearish to the center (say at 5% of the radius, so that we're not inside the central black hole)? The article says:

"...in fact, an atomic clock located in a galaxy could tick up to a third slower than the same clock in the middle of a void."

Maybe there is an nice simple formula assuming a uniform disk relating the time dilation to radius? I've always been under the impression that general relativity simulations needed supercomputers, but maybe current desktop machines are sufficiently "super" compared to 20 years ago that we could do simple calculations? Anyone have a recommendation on where to start learning enough general relativity to make a computer program? Assuming you've have a couple semesters of calculus-based college physics 101 under your belt as the prerequisite?

Free preprint fulltext:

Abstract: https://arxiv.org/abs/0912.4563

PDF: https://arxiv.org/pdf/0912.4563

JWST seems to be showing that more than a billion years of star and black hole evolution happened in the "first billion years". For about a year I've been pretty sure that the first billion years might have been more like ten billion years, maybe Timescape explains that.

This article really wants to sound sensationalist vs: "So all the stuff we used to ignore in our calculations, now that we are able to not ignore them it turns out that time dilation is acting in perfectly expected ways."

After hearing the explanation of timescale cosmology, it seems blindingly obvious. Of course mass can cause time dilation on large scales. How could it possibly be otherwise? Given a heterogenous universe, time must also be heterogeneous with respect to mass concentration.

It's just pure relativity, I can't believe we haven't already reached consensus on timescape over dark matter.

I really hope the theory stands up to scrutiny and that JWST and friends keep sending us new evidence. Timescape cosmology is just so neat that it must be true. Hopefully this will lead us in some new interesting directions

My son happens to be working on a flat spacetime variation of GR and that makes it much easier to think about these sorts of things.

To have flat spacetime you have to distort things other than spacetime -- other than space and time. Which things? Mainly the speed of light, but also there's length contraction (why? because the effects of gravity in GR are anisotropic). Thinking that the speed of light might be variable is very strange, but even in flat spacetime the speed of light is always the same locally (in all directions), just not globally (nor in all directions). So now think of the path a photon in the CMB took to get to us, and now trace it backwards: as you trace it backwards through a great void it speeds up! And as you trace its exit from a great void it slows down, but it slows down extra because now the density is higher than where we are (because the universe was denser in the past, and less dense now due to its expansion). This also means that there's different amounts of red-shifting when traversing different regions of the universe at wildly different times and differing wildly in density. There is even some blue-shifting due to traversal of great voids.

We use differences in attenuation of brightness of faraway standard type 1A supernovae candles (which we assume always have the same brightness locally at the time of the event) and their red-shift to infer acceleration of the expansion of the universe. (Assuming not much gravitational lensing we can expect attenuation of brightness of standard candles to be a simple function of distance, and then we'd expect the red-shift to agree, but if it doesn't then... if the red-shift is stronger we assume that extra red-shift to be due to the acceleration of the expansion of the universe.) But the differences in red-shift might be artifacts of the great voids that the light traversed in order to get to us. That's what this timescapes hypothesis is about! The difference in z might be explainable by great voids rather than by acceleration of the expansion of the universe -- or perhaps a combination of both even.

Flat spacetime is not actually incorrect. Einstein's field equations and its solutions (e.g., Schwarzchild's and Kerr's) are mappings between flat spacetime and curved spacetime. For example, the Schwarszchild and Kerr solutions for a single massive body (non-rotating, in the Schwarszchild case, rotating in the Kerr case) are expressed as curved spacetime second derivatives of time, space, and angles where there is an `r` variable which represents the flat spacetime distance to the center of the massive body. This is so much so that one could be forgiven for wondering if curved spacetime is not just a mathematical crutch, or a projection much like -say- a Mercator projection of a 3D map onto 2D. It sure does seem like flat spacetime is more fundamental than curved spacetime given that we have a flat spacetime `r` in the solutions.

I find it hard to believe that people who are working with the kind of the theories that require Dark Matter don't already account for time dilation due to general relativity.

The explanation made me think the universe is like one big zip file being extracted and in areas of lower entropy or complexity, it's able to be extracted faster. Maybe it's not an apt analogy but seems fitting as someone who analogizes tech to real world applications often.

What does timescape do to estimates of the age of the universe?

Random thought: does the idea that entanglement, time, and space being intimately linked have anything to do with this?

i'll wait to see what sean carroll thinks.

hard to believe no one found it before.