Scientists are considering whether the mysterious “force” accelerating the universe’s expansion changes with time.

Of late there’s been some scientific ado over a small but notable conflict in measurements of the universe’s expansion rate. The present rate, called the Hubble constant or H0 (pronounced “H-naught”), connects the redshift in an object’s spectra to its physical distance. It also tells us the universe’s age and size, as well as the density required to make the universe geometrically flat.

It is, in short, a rather important number.

Hubble constant measurements
This plot shows some representative measurements of the expansion rate of today's universe, known as the Hubble constant. The figure includes three values derived from measurements of the cosmic microwave background (left side of the plot) and four values derived from "nearby" objects (right side of plot). The values presented here are the bare-bones results from each study; any of these values can be made more precise by combining information from multiple data sets.
Ana Aceves / Monica Young

The universe’s expansion was one of the biggest discoveries of the 20th century. But after nearly a century of study, astronomers still don’t entirely agree on how fast the current expansion is. Those using the cosmic microwave background favor a value of about 67 kilometers per second per megaparsec (km/s/Mpc, where a megaparsec is 3.26 million light-years). Those using supernovae and other, more nearby cosmic tools home in on a value of about 73 km/s/Mpc (see graph).

The discrepancy is reminiscent of another debate toward the turn of the 21st century, with one side hotly advocating around 50 km/s/Mpc and the other roughly 100 km/s/Mpc. Careful work by Wendy Freedman (University of Chicago) and others using the Hubble Space Telescope resolved that controversy.

The current disagreement might be merely a matter of analysis and assumptions (maybe even a cosmic void). However, Freedman notes that part of the solution to the previous predicament was the mind-exploding discovery that, over the last half of cosmic history, the universe’s expansion has been accelerating. Something identified by the placeholder term “dark energy” fuels this acceleration. Although we don’t know what dark energy is yet, a favored view is that it’s some sort of energy inherent to space itself. If so, then as space expands, dark energy will increase with it and maintain the same density, instead of diluting out.

Could new physics be behind the current conflict, too?

Universe's expansion explained
The expansion of the universe began with the Big Bang, nearly 14 billion years ago. After a brief spurt of inflation, the expansion rate slowed, then several billion years later (estimates vary) acceleration kicked in when dark energy dominated gravity.
Nobel Prize Foundation

Gong-Bo Zhao (Chinese Academy of Sciences and University of Portsmouth, UK) and colleagues decided to explore this question. They investigated whether the latest controversy could be due to an evolving dark energy, one whose density not only changes with time but also changes in the way its density changes with time.

In mathematical terms, if dark energy’s density remains constant, then its equation of state, w, is equal to −1. This would make it the cosmological constant — the lambda (Λ) in the lambda-cold-dark-matter (ΛCDM) paradigm, the official name of our modern cosmological framework. Observations support this value, with wiggle room. If w is greater than −1 (−0.9, et cetera), however, then dark energy’s density slowly decreases as the universe expands. If w is less than −1, then the density increases with expansion.

What Zhao’s team did was to check if, instead of staying one value, the equation of state actually changes over cosmic time, morphing across a range of w values. Amassing a large, diverse set of others’ observations, the team cautiously analyzed it to see what kind of cosmos the data might describe. Writing August 28th in Nature Astronomy, the authors conclude that it is possible to relieve the H0 tension with a dynamic dark energy, one whose equation of state oscillates above and below the −1 value. However, the slight favoritism for this solution in their analysis is not statistically strong enough to prove it’s the right answer.

The nice thing is, more data will solve this mystery. The team points to the upcoming Dark Energy Spectroscopic Instrument (DESI) survey, which aims to begin creating a 3D cosmic map in 2018.


References: G.-B. Zhao et al. “Dynamical Dark Energy in Light of the Latest Observations.” Nature Astronomy. August 28, 2017.

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Peter Wilson

September 1, 2017 at 11:49 am

When are we going to a more realistic model of the universe?

Specifically, the universe is not just expanding. The universe is simultaneously collapsing, at all local scales: from meteors falling, to clouds of gas collapsing to form stars, to galaxies merging. This on-going collapse constitutes “action,” which causes a “reaction,” or expansion, in the universe-at-large. The reaction term should be included in the model, but it makes things complicated. An early “solution” to Einstein’s equations ignored the action/reaction part of the problem, in order to make the math manageable. The problem was reduced to just two variables, density and expansion rate, and of course, the acceleration of the expansion came as a complete surprise.

Yet twenty years later, there is still no “reaction” term in the standard model. Why not? The “problem” is unaccounted for acceleration of the expansion. Before adding an unknown “dark energy,” why not include all the known factors?!?

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September 3, 2017 at 6:27 am

Thankyou for your insight, worthy of a Nobel prize. Maybe if we added the collapsing universe into the models we could find all the dark matter. If "dark matter" has not been found already, does it exist at all? A collapsing gas cloud has to develop enormous particle density for metallic hydrogen to spontaneously begin fusion and we are expected to believe there is another source. I have to admit I am a "dark matter" sceptic, just show me the data and some actual proof before accepting any DM theory. DM could all be just photons and, I understand, they are bosons. PG

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September 6, 2017 at 7:01 am

Problem is that Dark Energy began to dominate at a time when the action was winding down (star-formation rates dropping, infrequent mergers, stunted growth of galaxy clusters etc.) and today there is not much action at all...

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Peter Wilson

September 7, 2017 at 8:28 pm

Not much action compared to what?!?

Until someone solves the equations, and puts the action and reaction onto mathematical footing, the dismissal means nothing.

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September 1, 2017 at 8:20 pm

Been wanting to bring this exact issue into the discussion here for a long time. Thank you for putting it so clearly.
Hope someone from S&T responds.

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Peter Wilson

September 4, 2017 at 12:49 pm

Thanks, to both of you. Basically, Sky&Tel only reports on what has been published, and what’s been published--lambda-CDM (dark energy + cold dark matter)--contains no action or reaction term, so that is why the issue is not discussed. But it would be nice if the editors became curious themselves, and wanted to know: What happens to the model when you add the reaction term(s)?

I cannot solve the equations of GR, so I can only pose the question, and guess at the answer. But the question is legitimate, and warrants an answer from officialdom.

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