Silence of the Lambda? Farewell, dark energy?

[TheVat]

Dark energy 'doesn't exist' so can't be pushing 'lumpy' universe apart, physicists say

One of the biggest mysteries in science—dark energy—doesn't actually exist, according to researchers looking to solve the riddle of how the universe is expanding.

phys.org

This timescape model is fascinating. When you read their explanation of different measures of time in the filaments, superclusters and voids, it makes you wonder what took so long to arrive at such a theory. It just makes sense. Large-scale structure, "lumpiness," has been known since what, the 80s? Here's a clip from the article:

For the past 100 years, physicists have generally assumed that the cosmos is growing equally in all directions. They employed the concept of dark energy as a placeholder to explain unknown physics they couldn't understand, but the contentious theory has always had its problems.

Now a team of physicists and astronomers at the university of Canterbury in Christchurch, New Zealand are challenging the status quo, using improved analysis of supernovae light curves to show that the universe is expanding in a more varied, "lumpier" way.

The new evidence supports the "timescape" model of cosmic expansion, which doesn't have a need for dark energy because the differences in stretching light aren't the result of an accelerating universe but instead a consequence of how we calibrate time and distance.

It takes into account that gravity slows time, so an ideal clock in empty space ticks faster than inside a galaxy....

 

[Gawdzilla Sama]

"First reports are always wrong." /sweepingeneralization

 

[Pinball1970]

Dark energy 'doesn't exist' so can't be pushing 'lumpy' universe apart, physicists say

One of the biggest mysteries in science—dark energy—doesn't actually exist, according to researchers looking to solve the riddle of how the universe is expanding.

phys.org

This timescape model is fascinating. When you read their explanation of different measures of time in the filaments, superclusters and voids, it makes you wonder what took so long to arrive at such a theory. It just makes sense. Large-scale structure, "lumpiness," has been known since what, the 80s? Here's a clip from the article:

For the past 100 years, physicists have generally assumed that the cosmos is growing equally in all directions. They employed the concept of dark energy as a placeholder to explain unknown physics they couldn't understand, but the contentious theory has always had its problems.

Now a team of physicists and astronomers at the university of Canterbury in Christchurch, New Zealand are challenging the status quo, using improved analysis of supernovae light curves to show that the universe is expanding in a more varied, "lumpier" way.

The new evidence supports the "timescape" model of cosmic expansion, which doesn't have a need for dark energy because the differences in stretching light aren't the result of an accelerating universe but instead a consequence of how we calibrate time and distance.

It takes into account that gravity slows time, so an ideal clock in empty space ticks faster than inside a galaxy....

Thought I'd posted this one! Must have been on another site. This is like dating two women, if you are putting some effort in you have remember what you did where or you could end up looking foolish. So I've heard.

 

[TheVat]

Thought I'd posted this one! Must have been on another site. This is like dating two women, if you are putting some effort in you have remember what you did where or you could end up looking foolish. So I've heard.

Maybe. I saw you posted a couple related threads here, one on Hubble tension, another on cold dark matter. This latest just came out in the past week. This is the first I've seen that just kicks dark energy to the curb, but I'm not exactly a panopticon.

 

[Pinball1970]

Maybe. I saw you posted a couple related threads here, one on Hubble tension, another on cold dark matter. This latest just came out in the past week. This is the first I've seen that just kicks dark energy to the curb, but I'm not exactly a panopticon.

Just checked, it was the other site. I was surprised with this although a paper came out regarding speed of light variability and also one on "tired" light which I thought had been debunked a while ago.

 

[exchemist]

I see Markus Hanke has posted a link to a very nice, readable explanation of how such "inhomogeneous cosmologies" can work, on the .net site. Here's the link: https://ncatlab.org/nlab/show/inhomogeneous+cosmology

I copy below what seems to me to be the essence of the idea:

In general, underdense regions [“voids”] are negatively curved and expand faster than the average, while overdense regions are positively curved and expand slower. (Räsänen 03, p. 15)
[...] as the volume occupied by [inhomogeneous] structures grows (along with the density contrast of typical structures), the expansion rate becomes dominated by voids, since their volume is large [...] overdense regions slow down more as their density contrast grows, and eventually they turn around and collapse to form stable structures. Underdense regions become ever emptier, and their deceleration decreases. Regions thus become more differentiated, and the variance of the expansion rate grows. (Räsänen 03, p. 25)
In an inhomogeneous space, different regions expand at different rates. Regions with faster expansion rate increase their volume more rapidly, by definition. Therefore the fraction of volume in faster expanding regions rises, so the average expansion rate can rise (Räsänen 10, p. 8)
The acceleration is not due to regions speeding up locally, but due to the slower region becoming less represented in the average. First the overdense region brings down the expansion rate, but its fraction of the volume falls because of the slower expansion, so eventually the underdense region takes over and the average expansion rate rises.
[...] After the overdense region stops being important, the expansion rate will be given by the underdense region alone, and the expansion will again decelerate. Acceleration is a transient phenomenon associated with the volume becoming dominated by the underdense region.
[...] Whether the expansion accelerates depends on how rapidly the faster expanding regions catch up with the slower ones, roughly speaking by how steeply the Ht curve rises. This is why the variance contributes positively to acceleration: the larger the variance, the bigger the difference between fast and slow regions, and the more rapidly the fast regions take over.
[...] So there is no ambiguity: accelerated average expansion due to inhomogeneities is possible. The question is whether the distribution of structures in the universe is such that this mechanism is realised (Räsänen 10, p. 10)

I note that Markus's comment is that whether such cosmologies can really do away with dark energy is highly dependent on the exact assumptions about inhomogeneity that are made. It seems to be very challenging to obtain enough observations to choose those assumptions based on real data and then to do the resulting calculations. But it might indeed work out.

I must say it does seem to be rather elegant. Not sure, though, where this would leave the QFT speculations about the possible role of zero point energy of the vacuum in accounting for dark energy. It would seem to leave its proponents rather stranded.

 

[Wizard of Whatever]

Could be echos of big bang.

 

[Pinball1970]

From Universe today. Supercomputer Simulations of the universe using variable Dark Energy versus Lambda CDM
You may need to watch a few times!

DESI, The Dark Energy Spectroscopic Instrument, has been specifically designed to map the universe and rates of expansion.

"DESI's first year of data tracked the expansion of the universe over 11 billion years. This initial data largely agreed with our Standard Model of Cosmology, called Lambda CDM. However, there were some small discrepancies suggesting that dark energy was changing over time. If it is changing, it's a direct challenge for the cosmological constant."

A simulated universe works better when dark energy changes over time

Dark energy is a mystery so daunting that it stretches and strains our most robust theories. The universe is expanding, driven by the unknown force that we've named Dark Energy. Dark Energy is also accelerating the rate of expansion. If scientists could figure out why, it would open up a whole...

phys.org

 

[Pinball1970]

There is a link to paper but is abstract only.

 

[Pinball1970]

So that is DESI, there is also Euclid.

Number 5 on the list.

Top five mysteries Euclid will help solve

Peering into the Universe beyond our galaxy, Euclid will try to untangle the mysteries of the cosmic web, and how invisible dark matter and exotic dark energy influence the structure and the course of the cosmos.

www.esa.int

Data released last week, the study will last six years. Imagining up to 10.5 Billion light years distant, the instrument has captured 26 million galaxies so far.

Euclid opens data treasure trove, offers glimpse of deep fields

On 19 March 2025, the European Space Agency's Euclid mission releases its first batch of survey data, including a preview of its deep fields. Here, hundreds of thousands of galaxies in different shapes and sizes take center stage and show a glimpse of their large-scale organization in the cosmic...

phys.org

 

[TheVat]

DESI's first year of data tracked the expansion of the universe over 11 billion years. This initial data largely agreed with our Standard Model of Cosmology, called Lambda CDM. However, there were some small discrepancies suggesting that dark energy was changing over time. If it is changing, it's a direct challenge for the cosmological constant."

I think astrophysics folk need to get humble and start admitting that the lambda is just an ad hoc term added in there to represent a conjecture. I appreciate they acknowledge the dangers of observational bias.

If the DESI result holds up, it means that a cosmological constant is not the origin of cosmic acceleration. It's much more exciting," said Hearin. "It would mean that space is pervaded by a dynamically evolving fluid with negative gravity, which has never been observed in any tabletop experiment on Earth."

IOW, "we have absolutely no idea what dark energy is or if it's even real or maybe there's some deep glitch in how we're interpreting our observational data. But 'dynamically evolving fluid with negative gravity' just sounded supercool so we had to put that in there."

 

[Pinball1970]

I think astrophysics folk need to get humble and start admitting that the lambda is just an ad hoc term added in there to represent a conjecture. I appreciate they acknowledge the dangers of observational bias.

If the DESI result holds up, it means that a cosmological constant is not the origin of cosmic acceleration. It's much more exciting," said Hearin. "It would mean that space is pervaded by a dynamically evolving fluid with negative gravity, which has never been observed in any tabletop experiment on Earth."

IOW, "we have absolutely no idea what dark energy is or if it's even real or maybe there's some deep glitch in how we're interpreting our observational data. But 'dynamically evolving fluid with negative gravity' just sounded supercool so we had to put that in there."

I would check the Euclid post right after it too, 26 million galaxies mapped already and it will be doing those over again "tens of times."

Dark Energy
Dark matter
Hubble tension
Early "impossible" galaxies

These instruments will shine on them.

 

[Pinball1970]

That should have said "shine a light on them "

 

[foghorn]

I need clarification here..

Are they saying dark energy is a property of this “ dynamically evolving fluid with negative gravity”?

Or are they saying the negative gravity is what we have been calling dark energy up until now?

Or do they mean something else?

"If the DESI result holds up, it means that a cosmological constant is not the origin of cosmic acceleration. It's much more exciting," said Hearin. "It would mean that space is pervaded by a dynamically evolving fluid with negative gravity, which has never been observed in any tabletop experiment on Earth."

 

[TheVat]

There's a coherence deficit in phrases like negative gravity. Given that the theory that brought us lambda in the first place is the one that treats ordinary gravity as a pseudoforce (curvature of spacetime), it is really hard to see how we can have a negative form of gravity that's a repulsive force and how that would affect spacetime. Bear in mind that lambda has always been a problem child of quantum field theory as well. It says empty space is defined by the vacuum state, which is composed of a collection of quantum fields. All these quantum fields exhibit fluctuations in their ground state (lowest energy density) arising from the zero-point energy existing everywhere in space. So these fluctuations should contribute to the lambda, but actual calculations give rise to an enormous vacuum energy with the values predicted exceeding observation by some 120 orders of magnitude, a discrepancy that has been called "the worst theoretical prediction in the history of physics!". And none of this really makes sense of why the energy density would produce a negative gravity, whatever TF sort of pseudoforce that is.

 

[foghorn]

I'm also sort of puzzled when people say the universe started as an explosion.
That, leaves me asking, what then brought about a later increase in the Cosmological Constant, that is, the later increase in the rate of recession.
'Explosions' don't start off slow and later increase in speed?
That's why the word expansion seems a better word to use. But, that still doesn't answer the later increase in the Cosmological Constant, rate of 'expansion.'

 

[Pinball1970]

I'm also sort of puzzled when people say the universe started as an explosion.
That, leaves me asking, what then brought about a later increase in the Cosmological Constant, that is, the later increase in the rate of recession.
'Explosions' don't start off slow and later increase in speed?
That's why the word expansion seems a better word to use. But, that still doesn't answer the later increase in the Cosmological Constant, rate of 'expansion.'

There is no suitable word for what happened at the beginning of the universe because it only happened once as far as we know, and the conditions were the most extreme ever achieved in the Universe.

As far as we know.

It would be like trying to find words for an atomic explosion if we had only ever experienced wood fire.

Explosion is used because it was very hot, 10 to the 32 Kelvin, Hiroshima was about 7,000K, centre of the sun about 15 million Kelvin and billions of kelvin in a supernova.

Here on earth TNT explodes at 500k Napalm burns at 1000K so between 100 - 1000 K

So, the early conditions we cannot fathom, and explosion does not cover it.

One misconception is that it started as a point, smaller than an atom.

Current thinking is the universe is flat and spatially infinite so it must have been infinite in the past, not easy to try and visualize if at all.
So, an infinite region of quark gluon plasma at approximately 10 to the 32 Kelvin expanding out in all directions very quickly. (I will not compare with earth shockwave hopefully you get my point)
Good news is that they can look for the shockwaves this created, baryon acoustic oscillations using DESI.


Let there be light? No, Let there be noise!

 

[foghorn]

One misconception is that it started as a point, smaller than an atom.

Current thinking is the universe is flat and spatially infinite so it must have been infinite in the past, not easy to try and visualize if at all.
So, an infinite region of quark gluon plasma at approximately 10 to the 32 Kelvin expanding out in all directions very quickly.

Good post.
I think that’s another reason to use expansion instead of explosion.
If I remember rightly, there is a way of not having to use the word ‘point’ for a space location, and so avoiding the classical explosion ‘point origin’ misunderstanding.

Run a spatially infinite universe backwards in time, and you reach a point in time when the whole universe is of ‘infinite density’.
But, really that is the time you start to look at changing your modelling equations.

 

[geordief]

Run a spatially infinite universe backwards in time, and you reach a point in time when the whole universe is of ‘infinite density’.
But, really that is the time you start to look at changing your modelling equations.

Are there any attempts at making those models?

I take it that it would be around the time T+10^-43 seconds?

Are there any forces at work then that might apply?

 

[Pinball1970]

Are there any attempts at making those models?

I take it that it would be around the time T+10^-43 seconds?

Are there any forces at work then that might apply?

Hawking Penrose and BGV probably do that in different ways but I will check with Linde when I get back and have a look.
Pop sci goes crazy with this sort of stuff because the goal posts keep moving so there are a lot of competing theories.

On the forces question, extrapolate far enough back and there is only one force, not the four fundamental forces we observe today.