[HN Gopher] Universe's expansion 'is now slowing, not speeding up'
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Universe's expansion 'is now slowing, not speeding up'
Author : chrka
Score : 62 points
Date : 2025-11-06 20:45 UTC (2 hours ago)
(HTM) web link (ras.ac.uk)
(TXT) w3m dump (ras.ac.uk)
| karakot wrote:
| What does 'now' mean here?
| plasticchris wrote:
| Probably it means that now we have evidence that... it is a
| colloquialism
|
| Edit: yep, The universe's expansion may actually have started
| to slow rather than accelerating at an ever-increasing rate as
| previously thought, a new study suggests.
| sermah wrote:
| Recent years, probably because of large data centers /s
| thelibrarian wrote:
| Going by the second graph, since about 2.5 billion years ago.
| 2OEH8eoCRo0 wrote:
| What happened to then?
| dylan604 wrote:
| we passed then. we're at now now. I thought this was settled
| jl6 wrote:
| My lay reading of the OP's paper is that the universe is, in
| fact, braking for somebody.
| denismenace wrote:
| Did it change during our life time?
| oofbey wrote:
| Just our understanding of it. That's flipped multiple times in
| my lifetime.
| candiddevmike wrote:
| Aside from unanswerable questions (has the universe started to
| fill it's container? Is a simulation property nearing "1"?), does
| this make long distance space travel feasible again? I thought
| there was something around the universe is expanding too fast to
| visit places like Alpha Centuri (and preventing visitors to us).
| indoordin0saur wrote:
| The universe was always only expanding between galaxies, not
| within them.
| Razengan wrote:
| So wait, individual stars aren't getting further apart?
| Galaxies aren't getting "bigger"/more diffuse?
| kmeisthax wrote:
| Galaxies have enough gravity to counteract the expansion of
| the universe.
| Razengan wrote:
| So do we see the expansion cancelled out by the gravity,
| or do we only see the gravity?
|
| I mean, is it change = gravity
|
| or change = expansion - gravity
|
| Because this just made me wonder.. is "dark energy"
| simply the _absence_ of gravity? i.e. just in regions
| where there is next to no matter /activity?
| oofbey wrote:
| That limitation only counts for visiting other galaxies. Travel
| within the galaxy is always possible, regardless of the
| universe's expansion. And Alpha Centauri is super close, even
| within our galaxy.
| dtech wrote:
| Specifically the local group, so Milky way + Andromeda and
| some dwarf galaxies
| Sharlin wrote:
| Dozens of dwarf galaxies, even! Also, Triangulum is sort of
| borderline at around 70% of the Milky Way's diameter,
| although admittedly only 10% of its mass. But Mars is also
| around 10% of Earth's mass, for a comparison.
| Sharlin wrote:
| Edit: A big brain fart, ignore the retracted part below.
| Colonizing the universe is of course impossible in 100My,
| barring FTL. What the paper I referred to [1] says is that
| colonizing the Milky Way may take less than that, and if you
| can do that, spreading to the rest of the observable universe
| is fairly easy, _very_ relatively speaking.
|
| <retracted> According to some calculations, it should in
| principle be possible to colonize the entire observable
| universe in less than a hundred million years. It's much too
| fast for the expansion to affect except marginally.</retracted>
|
| The relative jump in difficulty from interstellar to
| intergalactic is much smaller than from interplanetary to
| interstellar.
|
| Anyway, as others said, mere intragalactic (and intra-Local
| Group) travel is not affected by expansion in any way
| whatsoever.
|
| [1] https://www.sciencedirect.com/science/article/abs/pii/S0094
| 5..., PDF at
| https://www.aleph.se/papers/Spamming%20the%20universe.pdf
| hn_acc1 wrote:
| I found someone saying colonize the Milky Way Galaxy in ~90m
| years? Is that what you meant?
|
| The observable universe is ~93B LY - unless you're assuming
| FTL (and MUCH faster than light), I don't see how that's
| possible?
| Sharlin wrote:
| Yes, my brain totally froze. Added a correction.
| delta_p_delta_x wrote:
| > The relative jump in difficulty from interstellar to
| intergalactic is much smaller than from interplanetary to
| interstellar.
|
| Interesting way to put it... This doesn't seem that accurate.
| With sufficiently advanced technology, many of which we
| already possess, we could expect to propel a minute
| spacecraft to a considerable fraction of the speed of light,
| and reach nearby stars possibly within the end of the
| century. Reaching the other end of the galaxy is a massive
| undertaking. It's a logarithmic scale at every step of the
| way.
|
| Pluto is about 38 AU from Earth. Proxima Centauri is about
| 6.3 x 10^4 AU away (or about 4.24 ly), and that's roughly a 2
| x 10^3 multiplication. The Milky Way is about 50000 ly in
| radius, and the Andromeda Galaxy is about 3 x 10^6 ly away.
| Going from interplanetary distances to interstellar, and
| thence to intergalactic, involves at least a 10^5 factor
| (give or take) at each step.
| mkl wrote:
| If you can get to a star 100 light years away, you can get
| to Andromeda. It doesn't require going faster, just waiting
| longer.
| skissane wrote:
| I guess the question is... we know what our current
| propulsion technology is capable of... given a million
| years of further technological development, where will our
| technology be?
|
| The idea that, _given a million years of further
| technological development_ , intergalactic travel might
| actually be feasible, isn't really that implausible. Far
| from certain, but far from implausible either.
|
| And that's the thing-a million years is a technological
| eternity, a rounding error in estimates of time to colonise
| the galaxy/the local group/the observable universe.
| xoa wrote:
| > _According to some calculations, it should in principle be
| possible to colonize the entire observable universe in less
| than a hundred million years_
|
| ...what? That doesn 't seem right, just from a really quick
| gut check it looks like the observable universe has a radius
| of 45.7 billion light years [0]. Even if the universe wasn't
| expanding nobody could get to everything any faster than that
| number of years right? Maybe you saw something that was
| talking about the local (Virgo) supercluster, which I think
| has a radius of around 55 million light years, so that sounds
| more like something that could be done on that timescale "in
| theory". But there are millions and millions of superclusters
| in the observable universe overall.
|
| ----
|
| 0: https://en.wikipedia.org/wiki/Observable_universe
| palmotea wrote:
| >> According to some calculations, it should in principle
| be possible to colonize the entire observable universe in
| less than a hundred million years
|
| > ...what? That doesn't seem right, just from a really
| quick gut check it looks like the observable universe has a
| radius of 45.7 billion light years [0].
|
| I guess it depends on whose hundred million years you're
| talking about: the colonists' or those who stay home's. I
| don't know how to do the calculations, but it seems
| plausible that you could traverse the entire observable
| universe at near light-speed in 100 million years _ship
| time_.
| grvbck wrote:
| You need ridiculous speeds for time dilation to really
| kick in though. Mathematically, it starts as soon as an
| object moves. But if a spaceship travels at 90 % of light
| speed (0.9 c), their local time moves just approximately
| at half speed compared to local time on earth. A year for
| the astronauts is just over 2 years on earth.
|
| At 0.995 c, the ship clock runs 10 x slower.
|
| At 0.999 c, 22 x slower. Then if you push the turbo
| button to 0.9999 c, 71 x slower.
|
| The fastest man-made object to date is the Parker Solar
| Probe, at 0.059 c.
| Sharlin wrote:
| Oops, yes, I don't know what I was thinking. A total brain
| fart. The paper I referred to is Sandberg and Armstrong's
| 2012 "Eternity in Six Hours", and of course they don't
| claim such a thing. Only that it's possible to start a
| colonization wave that has plenty of time to spread to
| everything visible now before they slip outside of our
| future light cone. The ~100M years refers to the
| colonization of the Milky Way. Sorry!
|
| [1] https://www.sciencedirect.com/science/article/abs/pii/S
| 00945...
| jfengel wrote:
| The limit to space travel is the Rocket Equation, which says
| that you require exponential fuel to reach higher speeds. Alpha
| Centauri isn't going anywhere, but it will take millennia of
| travel even with wildly optimistic assumptions.
|
| Also note that there isn't any "container" to fill up. It could
| well be infinite. It's just that we will be forever limited to
| a finite subset, even in theory.
| redwood wrote:
| Was there a date at the top of this? I didn't see one. I saw a
| similar headlines earlier this year and I'm trying to understand
| that this is something new
| felixfurtak wrote:
| the linked journal article is dated Nov 6 2025
| observationist wrote:
| >>>Submitted by Sam Tonkin on Thu, 06/11/2025
|
| At the very bottom. Weird how style guides keep putting
| important information like this in harder to reach places.
| palmotea wrote:
| Is it SEO? IIRC there's a trend of removing dates from blog
| posts and articles, and my understanding it's to make the
| content seem more "evergreen" to Google (vs and article with
| a date, they may get down-ranked eventually due to age).
| samdoesnothing wrote:
| Thanks, AI.
| jimbo808 wrote:
| Anyone know how credible this is? If true, then that means the
| big bounce is back on the menu, and the universe could actually
| be an infinitely oscillating system.
| jampekka wrote:
| At least The Guardian has a comment from an independent expert:
|
| "Prof Carlos Frenk, a cosmologist at the University of Durham,
| who was not involved in the latest work, said the findings were
| worthy of attention. "It's definitely interesting. It's very
| provocative. It may well be wrong," he said. "It's not
| something that you can dismiss. They've put out a paper with
| tantalising results with very profound conclusions.""
|
| https://www.theguardian.com/science/2025/nov/06/universe-exp...
| HeinzStuckeIt wrote:
| As an academic, that is exactly what the kind of
| noncommittal, don't burn your bridges with colleagues and
| funding bodies thing that I would say about even clearly
| flawed research if I were put on the spot by a popular-press
| publication. In fact, if you know you can rebut flawed
| research in time, you might want to assist in hyping it first
| so that your rebuttal will then make a bigger splash and
| benefit your personal brand.
| observationist wrote:
| The more we learn, the less we end up knowing about how
| "everything" works - some things are mathematical in nature and
| demonstrate absolutes, but frameworks shift, and complexify,
| and exceptions to things we thought absolutes have occurred
| throughout history.
|
| For claims about how the universe works at scales and
| timeframes so utterly beyond anything testable, it's a little
| difficult to say this is credible at all - not dunking on the
| researchers, but in order to validate their conclusions,
| there's a whole chain of dependencies and assumptions you'd
| have to follow along with, and each of those things will be its
| own complex birds nest tangle of assertions, and I don't see
| how you can really say one way or another until you have a lot
| more information and a lot better Theory of Everything than
| we've got right now.
|
| For what it's worth, for all the impact it'll have on anyone's
| life outside of academia, I'd say they're 100% correct and
| people should buy them free beers at their local pubs for at
| least the next year in return for explaining their ideas at
| length.
| khimaros wrote:
| time to re-read "The Last Question"
| Cantinflas wrote:
| Asimov was so good. Amazing story
| CamperBob2 wrote:
| "Sorry, but as a large language model, I cannot provide
| advice on how to reverse entropy."
| pdonis wrote:
| _> If true, then that means the big bounce is back on the menu_
|
| I don't think so. Deceleration does not imply recollapse. AFAIK
| none of this changes the basic fact that there isn't enough
| matter in the universe to cause it to recollapse. The expansion
| will just decelerate forever, never quite stopping.
| pdonis wrote:
| _> Anyone know how credible this is?_
|
| AFAIK the previous models that all assumed that Type 1a
| supernovae were not affected by the age of the progenitor stars
| had no actual analysis to back that up; it was just the
| simplest assumption. This research is now actually doing the
| analysis.
| sfink wrote:
| If you cover up the part of the Figure 3 graph past "now", it
| kind of fits a sine wave.
| https://ras.ac.uk/sites/default/files/2025-10/Figure%203.jpg
|
| Universe gong.
| ertgbnm wrote:
| Seems like the problem should be pretty easy to figure out. Just
| need to wait ~5 gigayears and see which model is right. I'm
| personally hoping for deceleration so that we have more total
| visitable volume.
|
| I'll set a reminder to check back at that time to see who was
| right.
| aatd86 wrote:
| I would not be surprised if the universe was somewhat elastic,
| expands and then contracts and then expands ad infinitam. After
| all, existence in itself is irrefutable and cannot not exist by
| definition.
|
| If we subscribe to a theory of the multiverse, set theory,
| likelihood, and interaction driven evolution based on gradient
| type of fundamental laws. Locally changing. Obviously everything
| sharing a fundamental quality that is part of existence itself.
| But obviously there are sets, there is differentiation. But it is
| not created, the infinity of unconstrained possibilities exists
| in the first place and reorganizes itself a bit like people are
| attracted to people who share some commonalities or have
| something they need from each other and form tribes. Same
| processus kind of works for synapse connections, works for
| molecule formations, works for atoms... etc... Everything is
| mostly interacting data.
|
| We could say that the concept of distance is a concept of
| likelihood. The closer is also the most likely.
|
| Just a little weird idea. I need to think a bit more about it.
| Somewhat metaphysic?
| antonvs wrote:
| > After all, existence in itself is irrefutable and cannot not
| exist by definition.
|
| I can say the same about forgnoz, which is something I've just
| invented that must exist by definition.
|
| You'd need to try a bit harder to make existence actually
| inevitable.
| bombdailer wrote:
| Eventually we will find that the heat death of the universe and
| the big bang are the same thing, since the totality of the
| universe is always a oneness, then from the universal
| perspective the infinitely small and infinitely large are the
| same thing (one), then they by nature bleed into (and define)
| each other like yin and yang.
| antihipocrat wrote:
| You may appreciate this idea:
| https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology
| nabakin wrote:
| > type Ia supernovae, long regarded as the universe's "standard
| candles", are in fact strongly affected by the age of their
| progenitor stars.
|
| A key point in the article. From what I understand, this is the
| main way we measure things of vast distance and, from that,
| determine the universe's rate of expansion. If our understanding
| of these supernovae is wrong, as this paper claims, that would be
| a massive scientific breakthrough.
|
| I'm really interested in the counterargument to this.
| ardit33 wrote:
| Circular universe...? big bang -> expands -> expansion slows ->
| starts retracting -> singularity again -> big bang again
|
| Roger Penrose seems to be leaning/more convinced of the circular
| universe theory....
| johnwheeler wrote:
| Just because infinity is a hard thing to understand doesn't mean
| the universe is and has always been infinite.
| mrbluecoat wrote:
| I have a great deal of respect for the sciences but sometimes
| astronomy just feels like one giant guessing game: age of the
| universe, big bang starting as a joke and all the "first minute"
| timelines thereafter, dark energy and dark matter (code for we
| have no idea what it is) vastly outnumbering everything else, and
| now questioning the Nobel Prize-awarded universe expansion.
| Meanwhile, asteroids the size of buses+ keep whizzing by closer
| than the moon with little or no warning. Sigh.
| CamperBob2 wrote:
| That's a feature! If you want to be certain, you need religion,
| not science.
|
| And of course, the people concerned with tracking near-earth
| asteroids are not connected in any way with cosmology.
| dylan604 wrote:
| what? no. religion is not certain which is evidenced by the
| numerous sects of christianity with their own interpretations
| of the same book.
|
| while science might not have a definitive answer for
| everything, they distinguish from fact and theory.
| shomp wrote:
| Mainstream physics has been delighted to ignore/abandon essential
| conservation laws when talking about the expanding universe. It's
| kinda weird, I tried publishing a paper on it recently and it was
| not received well. In general, if conservation laws are to hold,
| expansion must be balanced with [eventual] contraction, is that
| not obvious? Apparently it was quite contentious to say until...
| this article?
| antognini wrote:
| Noether's theorem tells us when we would expect conservation
| laws to hold and when we would expect them to fail. In the case
| of global energy conservation, there would have to be a global
| time invariance associated with the spacetime. But this is
| manifestly not the case in an expanding universe. It is
| generally not even possible to have a well defined notion of
| global energy in a dynamic spacetime.
| zygentoma wrote:
| No, the assumption was that dark energy is a property of space
| itself so it does not conserve energy at all in an expanding
| space.
|
| Also this discovery does still is being explained with dark
| energy (albeit time varying ...) so it still does not assume
| global energy conservation.
| frotaur wrote:
| I mean no disrespect, but are you a trained physicist, or at
| least familiar with the 'mainstream material'?
|
| Because there is no shortage of 'crackpots' that have 'obvious'
| solutions to unsolved physics problems, and that want to
| publish papers about it.
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