From dark gravity to phantom energy: what's driving the expansion of the universe?

From dark gravity to phantom energy: what's driving the expansion of the universe?
There are two broad ways to measure the expansion of the universe. One is based on the cosmic microwave background, shown here, along with our own galaxy viewed in microwave wavelengths. Credit: ESA, HFI & LFI consortia (2010)

There is something strange happening in the local universe, with galaxies moving away from each other faster than expected.

What is driving this extra expansion, and what does it mean for the cosmos? To explore this, let's start with the observations.

The rate of cosmic expansion is encapsulated in the "Hubble constant", although don't let the name fool you, as it's not a constant and changes as the universe expands.

To determine this constant, astronomers must relate the distances to galaxies to the velocity they're travelling away from us. But measuring astronomical distances has always proven difficult. This is because we lack convenient signposts, known as standard candles and rulers, to chart the heavens.

So astronomers have built up cosmic distances through a series of steps, using overlapping methods to span the heavens. But each step in this cosmological distance ladder has its own quirks and uncertainties, and extraordinary effort over many decades has been expended to calibrate the various methods.

A new paper has pushed this calibration even harder, using a number of methods to tie down the Hubble constant to an accuracy of 2.4% within a few hundred million light years (which is local by cosmic standards).

A great success! But there's a problem.

We can also determine the universal expansion from observations of the cosmic microwave background, which is the radiation leftover from the Big Bang.

Unlike local observations, this reveals the global expansion of the universe. And this is where the problems begin, as this global expansion is 9% slower than that seen in the local universe. In both measurements, the astronomers have worked hard to reduce the uncertainties, and so are confident this difference is valid.

So what can explain this tension in cosmic measurement? Here are a few of the contenders.

Cosmic contenders

Dark matter

The first potential culprit is dark matter, the dominant mass in the universe. We know it is not smoothly spread through space, so perhaps the lumps and bumps, like the galaxies and clusters of galaxies, are exacting less gravitational pull in the local universe.

Perhaps we are in a cosmic void, a region whose density is below the universal average.

If this were the case, we would have to be inhabiting a strange corner of the universe, sitting at the centre of immense emptiness not very unlike anything expected in our cosmological ideas.

Dark energy

And then there is dark energy, the dominant energy in the universe. This component is responsible for accelerating the cosmic expansion, but is assumed to have a very simple form, eternal and unchanging over all of history.

But what if is dynamic and evolving, changing its properties as the universe expands? If it changed quite recently (in cosmic terms), the additional expansion could be imprinted on the local universe, but have not yet impacted the global expansion.

From dark gravity to phantom energy: what's driving the expansion of the universe?
A diagram representing the evolution of the universe, starting with the Big Bang to present day. The red arrow marks the flow of time. New research suggests it’s expanding even faster than shown here. Credit: NASA/GSFC

If this is the case, the universe has something to worry about, as this new form of dark energy would be a "phantom", driving universal expansion faster and faster into a "big rip", which is more dramatic than it sounds.

Dark radiation

Another potential solution is "dark radiation", which consists of hyper-fast particles that zipped around in the early universe.

While there is no single definition on what constitutes dark radiation, a favoured candidate is a new member of the neutrino family, affectionately known as sterile neutrinos.

While dark radiation is theoretical, there is little observational evidence for its existence. But if it had been present in the early universe, it would have influenced the early expansion of the universe, which would still be imprinted on the global value of the Hubble constant, but would now be washed out of the local value.

Dark gravity

The potential solutions so far have considered modifying the properties of components in the universe, but there is the more drastic alternative: dark gravity.

This suggests that we don't fully understand the fundamental nature of the , and that gravity does not follow the rules laid out by Albert Einstein in his general theory of relativity.

Such theories of modified gravity have existed for a long time, and come in many forms, and it is not clear how we deduce the impact of such gravity on the universal expansion.

Dark speculations

So there are several alternatives that could potentially explain the discrepancy between the local and global measurements of the Hubble constant. Which one is correct?

At the moment, the observations are rather raw and do not discriminate between the possibilities. And so we enter the realm of theoretical speculation, where ideas are tried and discarded until viable explanations are discovered.

At the same time, astronomers will seek more data, and will continue to tie down calibrations and methods. This brings us to our final possibility.

No observations are perfect, and much of science is about understanding the uncertainties of measurements. Scientists can generally wrangle random errors and understand how uncertainties in measurement impact uncertainties in results.

But there is another uncertainty: the systematic error, which can strike fear into a researcher. Instead of scattering results, systematic errors shift all results one way or another.

Systematic errors can also influence astronomical distance measures. And if they propagate through the distance ladder, they could potentially shift the local measurement of the Hubble constant away from the global value.

With new data and methods, this tension may evaporate. Some astronomers are already suggesting that this is a "more reasonable explanation".


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Hubble finds universe may be expanding faster than expected

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Jun 07, 2016
Hmm, dark everything? Sounds like it's time to uncover and re-discover Tesla's theory of gravity.

Jun 07, 2016
this is my theory - for years I have talked about it - gravity gets more strong as it travels there is not any dark matter all forces get more strong as they travel and this includes anti gravity if there is anti gravity Kurt Stocklmeir

Jun 07, 2016
A Universal black hole is powering our visible Universe.

'Black holes banish matter into cosmic voids'
http://www.spaced...999.html

"Some of the matter falling towards the [supermassive black] holes is converted into energy. This energy is delivered to the surrounding gas, and leads to large outflows of matter, which stretch for hundreds of thousands of light years from the black holes, reaching far beyond the extent of their host galaxies."

At the scale of our Universe the energy described above is dark energy. A Universal black hole is powering our visible Universe causing the galaxy clusters to accelerate away from us.

Jun 07, 2016
We are in the "dark ages" of Cosmology! Don't worry though, the Church of Astrophysics will ensure protection for the dark pseudoscience. They will continue with the Inquisition of the Dark Sciences to protect it from herecy and criticism of intelligent people.

Jun 07, 2016
Know what else is expanding faster than expected?
The ridiculousness in astronomy.

Jun 07, 2016
Such dark thoughts. More likely, what is driving the expansion is the fanciful thoughts of the committed merger maniac, ever bent on misinterpreting the data to conform to his limited but wishful thinking.

Jun 07, 2016
From the article "No observations are perfect," ... taken out of context this is rather telling as it explains all this mess. In other words: "anything that is not observable will always be perfect".

Jun 07, 2016
I think it's just 'human nature' kicking in. Find something we don't understand and can't classify and we get a bit confused. Then different people start to think and come up with various ideas and I don't see anything wrong in that. Rather like police detectives trying to solve a mysterious crime, many of them will want to pursue a different approach and the boss lets that happen hoping one will succeed. Scientists are detectives (of a sort) so let's hope one of the ideas on 'dark' problems sheds some light on the matter (wow, a pun?).

Jun 07, 2016
Personally I don't find a 9 % discrepancy in most astronomical measurements anything that needs a lot of speculation to explain.

Science by error bar and no explanation needed, bingo. When you're missing 96% of the Universe, 9% discerpency sounds pretty good, doesn't it.

Jun 07, 2016
Evidently that is very unlikely to ever happen.
wikipedia:

Ahh yes, the authority of all things- wiki....
The US government seized all of his belongings when he died, we may never learn much of his findings.

Jun 08, 2016
I have no opinion; I'll wait to see if they discover any systematic errors. With the JWST going up soon, this is likely to be well tested in the near future.

Jun 11, 2016
Make Pluto a Planet Pluto again and all will be right. Make it not, and the public will jerk the tax investment from the foundations that voted Pluto into purgatory. Then the NEW governing bodies, such as they will be, will give Planet Pluto its just place in our Solar System..... And we can go back to using the old textbooks.

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