
Cosmologist claims Universe may not be expanding (2013) - hairytrog
https://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379
======
micheles
I know Christoff Wetterich! From the time of my Ph.D. when we were working the
Renormalization Group. I met him a few times, we were working on similar
things but in different groups, we were "competitors" if you wish. Funny to
see him cited in HackerNews. He is certainly a respectable physicists, but
that can be said for many others. I have not read the cited article, it is in
my field, so I could understand it if my memory worked well, but after 20
years from my Ph.D. I am too rusted now.

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mpauly
The central concept behind Christof Wetterich's proposal is scale invariance:
Imagine you try to measure the size of the universe with a yard stick. If you
measure that the universe is expanding, you can't be sure that it really is
the universe expanding, or maybe just your yard stick shrinking.

This is true in so-called scale invariant theories. These theories do not
contain any intrinsic length scale. The moment your theory contains a
fundamental length l you break scale invariance. In this case you are able to
compare your yard stick to the length l to find out if your yard stick was
shrinking or the universe really is expanding.

Our present model of the world still contains a few fundamental length scales
(such as the Planck length, or a length scale associated to the nuclear
force), but people such as Christof Wetterich are building models that try to
get rid of (some of) these fundamental length scales. A major approach in this
direction is asymptotically safe quantum gravity, that also led Christof
Wetterich and Mikhail Shaposhnikov to the most precise prediction of the Higgs
mass prior to the Higgs' discovery in 2009, see
[https://arxiv.org/abs/0912.0208](https://arxiv.org/abs/0912.0208).

~~~
gubbrora
But the yardstick argument assumes uniform expansion. Small scale structure
like our yardstick is held together by electromagnetic forces which
counteracts the expansive force. That's my understanding anyway.

If we create a light year long line of yardsticks then we would expect the
line to expand faster than the yardsticks that make it up. Locally this would
look like growing gaps between the yardsticks. we never used the fact that the
line was a light year long. So let's just put two yardsticks next to each
other in a vacuum. They drift apart and eventually so far they are not part of
each others' observable universes.

~~~
Pharmakon
That’s accurate. Metric expansion only dominates loosely bound structures on a
scale beyond individual galalxies. The yardstick, solar system, and Milky Way
are all on scales too small, and are too strongly bound, for metric expansion
to have an impact. Galaxy clusters and the space between galaxies however, are
impacted.

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sanxiyn
This is (2013). Christof Wetterich continued research, his current theory is
called emergent scale symmetry:
[https://arxiv.org/abs/1705.00552](https://arxiv.org/abs/1705.00552). It got
peer-reviewed and published at Physical Reviews D, the leading journal on
cosmology.

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cmehdy
Interesting take on things. There's value in thinking about equivalent models
that allow potentially entrenched assumptions to be set aside. Worth
mentioning that the kilogram has since been defined more robustly than the way
mentioned in the article. [https://www.sciencealert.com/it-s-official-the-
definition-of...](https://www.sciencealert.com/it-s-official-the-definition-
of-a-kilogram-has-changed)

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mirimir
> The idea may be plausible, but it comes with a big problem: it can't be
> tested.

Doesn't that imply that the accepted model can't be tested? I mean, in the
sense of distinguishing it from this one.

~~~
oarla
Not a physicist, but I recall reading that there is enough empirical evidence
to support the current model of expanding universe.

~~~
DougN7
I always wonder about that. I remember very specifically learning 40(?) years
ago that the expansion was slowing, supposedly proven by some sort of red
shift in light. That is now out of favor, but a few more new facts just might
swing things the other way again.

~~~
sanxiyn
Accelerating universe is supported by supernova data. We have incomparably
more supernova data compared to the past. Discovery of accelerating universe
was mainly due to radically scaling up supernova discovery process.

~~~
madaxe_again
Yes, but that’s still dependent on a differential velocity model for the
origin of redshift. This paper changes that core assumption, meaning the same
data can be interpreted to have a radically different meaning.

Afaik, we have no direct observations of galaxies moving away from us - over
such enormous distances we have not had enough time since the start of
observations to actually see a galaxy recede.

~~~
gcthomas
No, the origin of the cosmological redshift is not proposed to be due to
differences in velocity, but metric expansion.

~~~
platz
... which causes a change in relative velocities

~~~
gcthomas
That makes it sound like cosmological redshift is a form of Doppler shift. In
reality, the target object emits light towards us with little Doppler shift as
it will have experienced little acceleration in its reference frame. The light
becomes redshifted as it moves through the universe while the universe is
expanding, effectively stretching out the wavelength of the light.

The change in wavelength is exactly proportional to the increase in size of
the universe during the time of flight, and it is not directly related to
speed of recession except in common and inaccurate discussion. The link to
relative velocity (if that concept makes sense for objects in widely separated
and unrelated reference frames) is model dependent, while the metric expansion
measurement is directly measured.

~~~
platz
Ok. I was under the impression that expansion had no effect on local phenomena
because local phenomena always counteract expansion. But if you view light
more as a wave instead of a particle then could see the argument that the
light is locally stretched if there is no force keeping it from being
stretched e.g. gravity or energy.

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KyleJ61782
Within galaxies, however, gravity dominates dark energy expansion, right? If
so, with enough measurement accuracy shouldn't we be able to look for standard
candles within our galaxy that either show or don't show said mass differences
while compensating for any shift due to relative motion? I'd think that given
enough accuracy we'd have something that's testable eventually.

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goombastic
Reminds me of my funny notes in a childhood diary. I had a personal pet theory
that Dinosaurs were so huge because they were not as heavy as we assumed them
to be. Imagining what it felt like to be a dino was a steady staple for boring
afternoons.

~~~
evanb
Maybe they're big because they're a lot closer than we realize.

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Geee
Wouldn't the gravitational pull increase? For example you would see less
gravitational lensing in older galaxy clusters.

~~~
pdonis
I think it's worse than that: it looks to me like this model will either not
predict what it's claimed to predict, or will violate local stress-energy
conservation.

Basically, if stress-energy is locally conserved, particle masses can't just
increase out of nowhere: the energy that goes into the increasing masses has
to come from somewhere, and so the overall energy, which is what drives the
dynamics of the universe, doesn't change, so the model can't predict anything
different from standard cosmology.

OTOH, if stress-energy is not locally conserved, then we should be able to
observe such violations of conservation. But no such observation has ever been
made: we have extremely strong evidence of local stress-energy conservation.

~~~
GlobalFrog
I can see your point, but is the introduction of dark energy in the current
favored model better? Instead of having an energy coming from nowhere that
increases particle mass, we have something coming from nowhere that expands
space.

~~~
pdonis
_> Instead of having an energy coming from nowhere that increases particle
mass, we have something coming from nowhere that expands space._

Dark energy does not "come from nowhere". It's locally conserved everywhere.
It has to be: the covariant derivative of the metric is identically zero, and
the "stress-energy tensor" of dark energy is just a constant times the metric.

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cairo_x
The universe isn't expanding. It's everything in it that's shrinking!

~~~
chr1
Strange that your comment is being downvoted, because it's a good summary of
the article!

For instance size of hydrogen atom is inversely proportional to the mass of
electron [1], so with heavier electrons, sizes of atoms shrink.

And in general in units where planks constant and speed of light are equal to
one, mass is measured in 1/length.

[1]
[https://en.m.wikipedia.org/wiki/Bohr_radius](https://en.m.wikipedia.org/wiki/Bohr_radius)

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largbae
This seems testable enough... The speed of light isn't in question here, so
the frequency is the yardstick. Can't we just record the frequencies given off
by the elements every hundred years or so and see that they are the same?

~~~
madaxe_again
That’s true, as with this cosmology light evidently isn’t affected by the
increase in rest mass, as photons are massless. If this cosmology affected
relativistic mass, then light would also be affected and would not be
seemingly redshifted by the mass gain mechanism.

So, yeah, we should be able to measure a gradual emission spectrum shift here
on Earth with current methods - we can measure energies of photons to very
high precision, and you’d likely only need a few years of data to prove or
disprove the hypothetical cosmology.

~~~
api
We should do it then. Maybe nobody ever looked because we assumed these things
to be invariant.

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booleandilemma
What are the theories on what we think the edge of the universe looks like?
What does it even mean? Will we just hit an invisible wall like in a video
game? Or do we think it’s curved around the edge? Or what?

If it’s expanding, what is it expanding into?

~~~
broth
This is something that has always intrigued me. What could possibly contain
the universe? Another universe perhaps?

~~~
brazzledazzle
I think such a thing is difficult to comprehend except maybe mathematically.
We are our universe and our universe is us.

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WeniTheElder
We observe from a far distant star the red shifted spectrum. This is explained
by the expansion of the galaxy and the expanding room between us and this
star. So the photons coming from there have a red-shifted and hence lesser
engery. What has happend to the energy of the photon? Where's it gone to?

~~~
greendestiny
I think the theory is suggesting that the photon was emitted with less energy.

~~~
jajag
I think the question is that if the current accepted theory for the redshift
is correct, then where has the energy gone?

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avsteele
I can't see why the article would say the effect is not measurable. We are
trying to contrast two explanations: In one, galaxies are receding and the
expansion of space in the intervening distance red shifts the spectra. In the
other masses are changing (globally?) as some rate, changing the spectra.

The best modern atomic clocks depend on atomic spectra, a shift in which would
be evidence for or against this model.

Given the precision of modern atomic clocks (<10^-17)[1] and the fact that the
galactic spectral shifts are significant at large distances this seems
plausible. I don't have precise #'s to do the calculations at hand so maybe
I've made an error.

[1]
[https://en.wikipedia.org/wiki/Quantum_clock](https://en.wikipedia.org/wiki/Quantum_clock)

~~~
AnimalMuppet
But if our atomic clocks' frequency is changing, how would you tell? You can't
use a more precise clock; we don't have one. You'd have to do something like
measure how far light travels in the same time. But if our length measurements
are changing too... do they cancel? Or could we still tell?

~~~
jepler
What you would need to determine is whether a dimensionless quantity is
changing.

[https://en.wikipedia.org/wiki/Fine-
structure_constant#Is_the...](https://en.wikipedia.org/wiki/Fine-
structure_constant#Is_the_fine-structure_constant_actually_constant)?

If the fine structure constant α is found to change, then would only be able
to say that the relationship among the speed of light c, the elementary charge
e, and the Planck constant ħ are changing, but IMO if we discovered that (say)
α changed from .007300… to its present value of .007297…, we would not be able
to then say "the speed of light changed (and the elementary charge and Planck
constant remained the same)"

…but as a matter of practicality, scientists could feel free to _use_ a
different value for c in the past (and the present value for the elementary
charge and Planck constant), if that is the most convenient way to accommodate
a change in α. It wouldn't surprise me in the least if holding c and ħ
constant is better for astronomers and holding e and ħ constant are better for
particle physicists.

(Saying that ħ changes would be similar to saying that mass changes, or at
least, the units of ħ include mass, and the Planck constant "is the basis for
the definition of the kilogram." in the SI system [well, technically, this is
not the case until next month. Right now the kilogram is still defined by the
physical "international prototype kilogram" object, but the above quote is
from wikipedia])

However, while the best experiments are consistent with changing α, the change
in α is constrained to be WAY less than it would need to be to explain
cosmological red shift observations.

------
edem
There is also the opposite scenario, the Big Rip:
[https://www.youtube.com/watch?v=gEyXTQ9do-c](https://www.youtube.com/watch?v=gEyXTQ9do-c)

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mmanfrin
Kind of related to something that I've been chewing on:

I've read things (outside of the OP) that say the universe is expanding and
also speeding up it's expansion, as neighbors exert less of a pull on each
other the further spread out things get. However, that gravitational pull is
still there -- will it at any point slow down the expansion and retract back
inwards?

~~~
pdonis
_> that gravitational pull is still there -- will it at any point slow down
the expansion and retract back inwards?_

No. If we are talking about the current best model we have of the universe,
the dark energy density (which is what drives the acceleration of the
expansion) is constant in time, while the density of matter (which is what
causes the expansion to decelerate) decreases with time. Up until a few
billion years ago, the expansion was decelerating, because the density of
matter was high enough to dominate. But since a few billion years ago, the
density of matter has been small enough that the dark energy dominates, and
that is never expected to change.

~~~
SiempreViernes
Though to be fair, our expectations on what dark energy will do aren’t very
well grounded.

We already have inflation and dark energy but no good reason to expect either
of them to be there _at all_. Like, the book on dark energy I’ve seen go:
”here are a bunch of pretty good guesses”, thats not a field full of
confidence in its expectations (though admittedly the book is 9 years old by
now).

~~~
pdonis
_> We already have inflation and dark energy but no good reason to expect
either of them to be there at all._

I have seen plenty of heuristic arguments for why we should expect inflation
to be there, but I'm not really qualified to judge. None of them seem to have
convinced enough physicists to be considered standard.

For the cosmological constant, I think it depends on what you consider the
"natural" condition to be. The simplest way to derive the Einstein Field
Equation is to start from the Einstein-Hilbert Lagrangian and vary it with
respect to the metric. The justification for using the Einstein-Hilbert
Lagrangian is that it contains the only scalar which contains up to second
derivatives of the metric and is quadratic in the metric and its derivative,
namely, the Ricci scalar. But that statement is not actually true: there's
another obvious scalar that meets this requirement, namely a constant. So the
most "natural" version of the Einstein Field Equation contains the
cosmological constant: it should be there on theoretical grounds, and that
means "dark energy" should be there as well.

Of course that also raises another problem, which is indeed one we don't have
a good answer to at present: why is the cosmological constant so small?

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mjfern
Wouldn't this theory rule out dark energy since I believe dark energy is used
to explain an accelerating, expanding universe?

~~~
chrispeel
Yes, I think that's the point.

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_bxg1
> Conversely, if the particles were to become lighter, the frequencies would
> become redshifted.

> If all masses were once lower, and had been constantly increasing, the
> colours of old galaxies would look redshifted

Can anyone explain this apparent contradiction?

~~~
lkrubner
If all masses were once lower, and had been constantly increasing, then those
galaxies are now blue-shifted, and blue shifted is what we have come to think
of as normal. But billions of years ago, when they were lighter, they were
emitting photons that, to us now, in the present, seem red-shifted. They seem
redshifted because they come from the past, when things were lighter.

~~~
Dylan16807
Or in very straightforward terms:

lightweight is red, heavy is blue

If everything constantly grows heavier then the past is red and the future is
blue.

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Abishek_Muthian
>Instead, the Big Bang stretches out in the past over an essentially infinite
period of time.

So the timeline according to this theory is [Big Bang] - >[Inflation] -
>[Increase in mass] & we're seeing the redshift from that?

Head hurts, I love it.

------
NoblePublius
Alvie would be pleased to know that Brooklyn is not expanding.

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y04nn
Does this mean that G is not contant? And would it throw away all our
calculations of distances of galaxies in the known Univers?

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RobertoG
We know there is an equivalence between mass an energy.

Does this view imply that new energy is appearing in the universe?

~~~
ccvannorman
The _traditional_ view is that the universe is expanding due to "dark energy",
which does presume that yes, new energy is appearing in the universe all the
time (at extremely low amounts, but over galactic distances it adds up to
cause expansion)

~~~
RobertoG
I suppose you are right, I didn't though about that. I find the implications
surprising.

Does that means that the universe can't be considered a closed system and
entropy increasing is not unavoidable?

------
revskill
As my imagination, the Big Bang is the collision of two universes in the past.

~~~
eesmith
"The ekpyrotic theory hypothesizes that the origin of the observable universe
occurred when two parallel branes collided." \-
[https://en.wikipedia.org/wiki/Brane_cosmology#Models_of_bran...](https://en.wikipedia.org/wiki/Brane_cosmology#Models_of_brane_cosmology)

~~~
revskill
Only a collision between super massive materials could produce such huge
larger energy as we see in Big Bang.

~~~
eesmith
Like a cyclical interaction with a neighbor brane which is essentially another
universe? Because that's the ekpyrotic universe model. And it fits with your
imagination.

~~~
revskill
What is "ekpyrotic" ? Google Translate can't translate it, sorry.

~~~
eesmith
The first hit from DuckDuckGo for 'ekpyrotic' is the Wikipedia page
[https://en.wikipedia.org/wiki/Ekpyrotic_universe](https://en.wikipedia.org/wiki/Ekpyrotic_universe)
. It describes the reason Steinhardt created that word as the name for the
model.

The Steinhardt and Turok preprint is at [https://arxiv.org/abs/hep-
th/0111030](https://arxiv.org/abs/hep-th/0111030) , published in the journal
Science at
[http://science.sciencemag.org/content/296/5572/1436.full](http://science.sciencemag.org/content/296/5572/1436.full)
.

> We propose a cosmological model in which the universe undergoes an endless
> sequence of cosmic epochs each beginning with a ‘bang’ and ending in a
> ‘crunch.’ The temperature and density are finite at each transition from
> crunch to bang. Instead of having an inflationary epoch, each cycle includes
> a period of slow accelerated expansion (as recently observed) followed by
> slow contraction. The combination produces the homogeneity, flatness,
> density fluctuations and energy needed to begin the next cycle.

See also
[https://en.wikipedia.org/wiki/Paul_Steinhardt#Cyclic/ekpyrot...](https://en.wikipedia.org/wiki/Paul_Steinhardt#Cyclic/ekpyrotic_theory_of_the_universe)
.

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fareesh
What does the Universe expand into?

~~~
oblomovshchina
Nothing. The concept seems foreign because we're always in something, but
there is definitionally nothing the Universe can expand into. If it's easier
to conceptualize, just think of 'expansion' as a short way of saying
'distances are getting longer'.

~~~
fareesh
So does it mean that everything in the Universe is getting bigger but since
everything is getting bigger we don't know it because relative sizes are the
same?

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castis
could this help explain the mass of the several copies of the kilogram
drifting apart?

