Can we ditch dark energy by better understanding general relativity?

June 29, 2017 by David Wiltshire And Alan Coley, The Conversation
Simulated universe. Credit: EAGLE collaboration, J Schaye et al 2015. MNRAS, CC BY-SA

A renewed suggestion that dark energy may not be real—dispensing with 70% of the stuff in the universe—has reignited a longstanding debate.

Dark energy and dark matter are theoretical inventions that explain observations we cannot otherwise understand.

On the scale of galaxies, gravity appears to be stronger than we can account for using only particles that are able to emit light. So we add as 25% of the mass-energy of the Universe. Such particles have never been directly detected.

On the larger scales on which the Universe is expanding, gravity appears weaker than expected in a containing only particles – whether ordinary or dark matter. So we add "": a weak anti-gravity force that acts independently of matter.

Brief history of "dark energy"

The idea of dark energy is as old as itself. Albert Einstein included it when he first applied relativity to cosmology exactly 100 years ago.

Einstein mistakenly wanted to exactly balance the self attraction of matter by anti-gravity on the largest scales. He could not imagine that the Universe had a beginning and did not want it to change in time.

Almost nothing was known about the Universe in 1917. The very idea that galaxies were objects at vast distances was debated.

Einstein faced a dilemma. The physical essence of his theory, as summarised decades later in the introduction of a famous textbook is:

Matter tells space how to curve, and space tells matter how to move.

That means space naturally wants to expand or contract, bending together with the matter. It never stands still.

This was realised by Alexander Friedmann who in 1922 kept the same ingredients as Einstein. But he did not try to balance the amount of matter and dark energy. That suggested a model in which universes that could expand or contract.

Further, the expansion would always slow down if only matter was present. But it could speed up if anti-gravitating dark energy was included.

Since the late 1990s many independent observations have seemed to demand such accelerating expansion, in a Universe with 70% dark energy. But this conclusion is based on the old model of expansion that has not changed since the 1920s.

Standard cosmological model

Einstein's equations are fiendishly difficult. And not simply because there are more of them than in Isaac Newton's theory of gravity.

Unfortunately, Einstein left some basic questions unanswered. These include – on what scales does matter tell space how to curve? What is the largest object that moves as an individual particle in response? And what is the correct picture on other scales?

These issues are conveniently avoided by the 100-year old approximation—introduced by Einstein and Friedmann—that, on average, the Universe expands uniformly. Just as if all could be put through a blender to make a featureless soup.

This homogenising approximation was justified early in cosmic history. We know from the cosmic microwave background—the relic radiation of the Big Bang—that variations in matter density were tiny when the Universe was less than a million years old.

But the universe is not homogeneous today. Gravitational instability led to the growth of stars, galaxies, clusters of galaxies, and eventually a vast "cosmic web", dominated in volume by voids surrounded by sheets of galaxies and threaded by wispy filaments.

An artist’s impression shows the European Extremely Large Telescope (E-ELT) which uses CODEX as an optical, very stable, high spectral resolution instrument. Credit: ESO/L. Calçada, CC BY-SA

In standard cosmology, we assume a background expanding as if there were no cosmic structures. We then do computer simulations using only Newton's 330-year old theory. This produces a structure resembling the observed in a reasonably compelling fashion. But it requires including dark energy and as ingredients.

Even after inventing 95% of the energy density of the universe to make things work, the model itself still faces problems that range from tensions to anomalies.

Further, standard cosmology also fixes the curvature of space to be uniform everywhere, and decoupled from matter. But that's at odds with Einstein's basic idea that matter tells space how to curve.

We are not using all of general relativity! The standard model is better summarised as: Friedmann tells space how to curve, and Newton tells matter how to move.

Enter "backreaction"

Since the early 2000s, some cosmologists have been exploring the idea that while Einstein's equations link matter and curvature on small scales, their large-scale average might give rise to backreaction – average expansion that's not exactly homogeneous.

Matter and curvature distributions start out near uniform when the universe is young. But as the cosmic web emerges and becomes more complex, the variations of small-scale curvature grow large and average expansion can differ from that of standard cosmology.

Recent numerical results of a team in Budapest and Hawaii that claim to dispense with dark energy used standard Newtonian simulations. But they evolved their code forward in time by a non-standard method to model the backreaction effect.

Intriguingly, the resulting expansion law fit to Planck satellite data tracks very close to that of a ten-year-old general relativity-based backreaction model, known as the timescape cosmology. It posits that we have to calibrate clocks and rulers differently when considering variations of curvature between galaxies and voids. For one thing, this means that the Universe no longer has a single age.

In the next decade, experiments such as the Euclid satellite and the CODEX experiment, will have the power to test whether cosmic expansion follows the homogeneous law of Friedmann, or an alternative backreaction model.

To be prepared, it's important that we don't put all our eggs in one cosmological basket, as Avi Loeb, Chair of Astronomy at Harvard, has recently warned. In Loeb's words:

To avoid stagnation and nurture a vibrant scientific culture, a research frontier should always maintain at least two ways of interpreting data so that new experiments will aim to select the correct one. A healthy dialogue between different points of view should be fostered through conferences that discuss conceptual issues and not just experimental results and phenomenology, as often is the case currently.

What can general relativity teach us?

While most researchers accept that the backreaction effects exist, the real debate is about whether this can lead to more than a 1% or 2% difference from the mass-energy budget of standard cosmology.

Any backreaction solution that eliminates dark must explain why the law of average expansion appears so uniform despite the inhomogeneity of the cosmic web, something standard cosmology assumes without explanation.

Since Einstein's equations can in principle make space expand in extremely complicated ways, some simplifying principle is required for their large-scale average. This is the approach of the timescape cosmology.

Any simplifying principle for cosmological averages is likely to have its origins in the very early Universe, given it was much simpler than the Universe today. For the past 38 years, inflationary universe models have been invoked to explain the simplicity of the early Universe.

While successful in some aspects, many models of inflation are now ruled out by Planck satellite data. Those that survive give tantalising hints of deeper physical principles.

Many physicists still view the Universe as a fixed continuum that comes into existence independently of the matter fields that live in it. But, in the spirit of relativity – that space and time only have meaning when they are relational – we may need to rethink basic ideas.

Since time itself is only measured by particles with a non-zero rest mass, maybe spacetime as we know it only emerges as the first massive particles condense.

Whatever the final theory, it will likely embody the key innovation of general relativity, namely the dynamical coupling of and geometry, at the quantum level.

Explore further: Explaining the accelerating expansion of the universe without dark energy

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xinhangshen
1.4 / 5 (9) Jun 29, 2017
Please be aware that Einstein's relativity is wrong because of the existence of the absolute time as shown by the universally synchronized clocks on the GPS satellites which are moving at huge velocities relative to each other and are synchronized not only relative to the ground clocks but also to each other (see Wikipedia on GPS), while special relativity claims that time is relative and are different in different reference frames.
baudrunner
5 / 5 (8) Jun 29, 2017
Please be aware that Einstein's relativity is wrong because of the existence of the absolute time as shown by the universally synchronized clocks on the GPS satellites which are moving at huge velocities relative to each other and are synchronized not only relative to the ground clocks but also to each other (see Wikipedia on GPS), while special relativity claims that time is relative and are different in different reference frames.
are you or are you not aware that accommodations to those clocks needed to be made to account for the relative time frames of the satellites in the absence of gravity and in relative motion to each other and the ground? What "absolute" time are you talking about, when each of those satellites, and the ground, occupy inertial frames of reference that are temporally unique?
TimLong2001
5 / 5 (3) Jun 29, 2017
All motions at the quantum scale operate in accordance with particles' natural interactions. At large scales we use orbits to set reference cycles such as hour, day, year, etc. This metrical representation of the current state of the universe (all that is actually real) is what we refer to as "Time" -- a tool we use to reference our activities, not an actual dimension.
humy
5 / 5 (7) Jun 30, 2017
Please be aware that Einstein's relativity is wrong because of the existence of the absolute time as shown by the universally synchronized clocks on the GPS satellites which are moving at huge velocities relative to each other and are synchronized not only relative to the ground clocks but also to each other (see Wikipedia on GPS), while special relativity claims that time is relative and are different in different reference frames.

NO, you are clearly very confused; the need to synchronize clocks on the GPS satellites is a result of time dilation effects just as relativity predicts thus proving time is RELATIVE, NOT absolute. Wikipedia link on GPS confirms this. You strangely got all of that back-to-front.
Urgelt
5 / 5 (5) Jun 30, 2017
GPS satellite clocks are time-dilated relative to Earth receivers, twice. There is a velocity difference, which slows satellite clocks relative to Earth receivers (special relativity); and the satellites are shallower in Earth's gravity well, which speeds their clocks relative to Earth receivers (general relativity). GR time dilation dominates, but both adjustments must be made to derive an accurate position for a receiver.

All of the satellites are at the same depth in Earth's gravity well, so satellite-to-satellite clock comparisons are ruled by SR.

This is not to say that GPS doesn't raise interesting questions, particularly with respect to the Einstein versus Lorentz predictions for time dilation. Einstein said time dilation effects are always reciprocal. Lorentz believed they might be nonreciprocal. Reciprocity is one area of relativity theory that has not been thoroughly tested, mainly because most physicists today dismiss the idea of nonreciprocity as silly.
antialias_physorg
5 / 5 (3) Jun 30, 2017
All of the satellites are at the same depth in Earth's gravity well

I wonder if there are more adjustments one could make. The GOCE mission has measured the Earth gravity profile (Earth is not uniformely dense...which means that even if satellites are at the same height they are not in an identically strong gravitational field.). It's a very minor effect, but still: for accuracy every bit counts.

(additionally one could make adjustment due to the sun's field gradient between near and far side or the Moon's effect)
TimLong2001
1 / 5 (2) Jun 30, 2017
The interaction of the internal charges of the binary (dipole) photon structure provide the energy for its self-propagation at the speed of light. This gradual entropy results in an increasing orbital radius and longer wavelength resulting in the background redshift -- sans expansion.
Urgelt
5 / 5 (1) Jun 30, 2017
Auntie, I'm sure you are entirely correct. But building clocks that can compensate on the fly for extremely subtle shifts in gravity fields experienced by a satellite would be difficult, for not very much gain in terms of location precision. Diminishing returns, you see?
Seeker2
not rated yet Jun 30, 2017
The interaction of the internal charges of the binary (dipole) photon structure provide the energy for its self-propagation at the speed of light.
Could it be the energy of propagation for photons comes from virtual electron-positron particle pairs. In other words the photon sets up a resonating wave of virtual positron-electron pairs. I presume the energy for virtual particle pair production is provided by the dark energy.
Seeker2
not rated yet Jun 30, 2017
...building clocks that can compensate on the fly for extremely subtle shifts in gravity fields experienced by a satellite would be difficult, for not very much gain in terms of location precision. Diminishing returns, you see?
Well maybe maybe not. Clocks at various altitudes could give you a like a 3d view into the internal structure of the earth. Or at least the variations of the internal structure.
Seeker2
not rated yet Jul 01, 2017
...Clocks at various altitudes could give you a like a 3d view into the internal structure of the earth. Or at least the variations of the internal structure.
Probably would be best to do this to survey the moon for water or gold or whatever before sending in the big mining machines. Could be more efficient than fleets of rovers. We don't need to trash up earth orbits any more than they already are now anyway.
Seeker2
not rated yet Jul 01, 2017
cont
Would be like getting a holographic image of the internal structure of the moon.
Da Schneib
5 / 5 (2) Jul 01, 2017
Dark energy is going to be harder to get rid of than dark matter, actually. Particularly by means of changes to relativity. This is because we can see that there is a "knee" in the curve of redshift to distance about 7 billion light years away (or, if you prefer, about 7 billion years ago). Kinda hard to think how relativity might have changed that way.
xinhangshen
1 / 5 (1) Jul 01, 2017
I would like to answer those questioned my claim above. I agree that we indeed need corrections of the atomic clocks on the GPS satellites, but please be aware that these corrections are absolute, not relative as claimed by special relativity. After the corrections, all the clocks are universally synchronized, which directly disproves special relativity. Continue in the next comment.

xinhangshen
1 / 5 (1) Jul 01, 2017
Some people may argue that the clocks are only synchronized in the earth centered inertial reference frame, and are not synchronized in the reference frames of the GPS satellites. If it were true, then the time difference between a clock on a GPS satellite and a clock on the ground observed in the satellite reference frame would grow while the same clocks observed on the earth centered reference frame were still synchronized. If you corrected the clock on the satellite when the difference became significant, the correction would break the synchronization of the clocks observed in the earth centered frame. That is, there is no way to make a correction without breaking the synchronization of the clocks observed in the earth centered frame.
Seeker2
not rated yet Jul 01, 2017
After the corrections, all the clocks are universally synchronized, which directly disproves special relativity.
GR and SR concern how fast clocks run, not how well they are synchronized.
Da Schneib
5 / 5 (1) Jul 01, 2017
@Seeker GRT and SRT tell how to calculate the speeds clocks run at under different conditions; GRT tells about different gravity, and SRT tells about different speeds.

OTOH, whoever you're replying to doesn't have it right either; the statement that clocks can be universally synchronized is also incorrect because it would require defining a universally correct time which doesn't exist, and the statement that the fact that the corrections between different clocks under different conditions can be rationalized to one another is exactly proof of relativity.
Seeker2
not rated yet Jul 01, 2017
@Seeker GRT and SRT tell how to calculate the speeds clocks run at under different conditions; GRT tells about different gravity, and SRT tells about different speeds.
All of which determines how fast clocks run. Don't confuse the poor guy about cause and effect.
Ensign_nemo
not rated yet Jul 02, 2017
The atomic clocks aboard the GPS satellites are adjusted to account for both special and general relativity. I once worked at the company that built the navigational payload for the Block II sats.

In practice, there are other effects. Some satellites are launched as backups and moved into orbital slots when other, older satellites fail. The new position of the satellites requires minor tweaks to the clocks.

The cesium and rubidium atomic clocks used also tend to drift a little bit as they age, so manual readjustments are made occasionally to resynchronize them.

Once in a while - every decade or so - the rotation of the Earth slows down by a second, so a 'leap second' is added at 23:59:60 on December 31 to resynchronize our clocks with our planet, and that requires another manual adjustment.

There are lots of practical considerations as well as relativistic effects that require resynching the clocks.
xinhangshen
not rated yet Jul 11, 2017
The atomic clocks aboard the GPS satellites are adjusted to account for both special and general relativity. I once worked at the company that built the navigational payload for the Block II sats.

Your experience in setting GPS clocks does not mean you understand special relativity. Actually most mainstream physicists don't understand special relativity and wrongly attribute the corrections of the clocks as relativistic effects. Please be aware that velocity caused time dilation in special relativity is different relative to different observer, which can never be corrected to make the clocks universally synchronized, while the reality is that the atomic clocks on the GPS satellites are universally synchronized.
nikola_milovic_378
not rated yet Jul 26, 2017
What are your watches made and manage time with them? If they are mechanical, time can not force the spring to change its behavior. If they are digital, the time again can not be affected by the drive (batteries). If atomic clocks are, can time influence the structure of the atom? If it is, then the structure of matter in celestial bodies, moving faster, is different and not like this on our planet. What is the intent of these unconscious people, to embarrass even the little awareness with which it disposes of.
First, I have to say this:
nikola_milovic_378
not rated yet Jul 26, 2017
You must all, understand and accept that the universe is an infinite sphere, filled with an ether from which matter is formed. This is what you call "dark" to the ether, just need to know how it is formed of matter and how it affects the "liquid" and "solid" state of matter, when it comes to relationships occur here gravitation and magnetism. Is there anyone smart to give up Einstein mirage and accept the existence of spiritual entities of the universe (SEU), who is the creator of everything composed of matter (that includes me, human beings).

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