Stuck in the middle with the Universe. Cosmic expansion lies in-between estimates

New measurements using Red Giant stars show that the Hubble Constant — the rate at which the Universe is expanding — lies somewhere between conflicting estimates.

Estimates of the rate of expansion of the Universe and the true value of the Hubble constant is quickly becoming one of the hottest debated questions in science. The best current methods of taking this value are in conflict.

These galaxies are selected from a Hubble Space Telescope program to measure the expansion rate of the universe, called the Hubble constant. The value is calculated by comparing the galaxies’ distances to the apparent rate of recession away from Earth (due to the relativistic effects of expanding space). By comparing the apparent brightnesses of the galaxies’ red giant stars with nearby red giants, whose distances were measured with other methods, astronomers are able to determine how far away each of the host galaxies are. This is possible because red giants are reliable milepost markers because they all reach the same peak brightness in their late evolution. And, this can be used as a “standard candle” to calculate distance. Hubble’s exquisite sharpness and sensitivity allowed for red giants to be found in the stellar halos of the host galaxies. The red giants were searched for in the halos of the galaxies. The centre row shows Hubble’s full field of view. The bottom row zooms even tighter into the Hubble fields. The red giants are identified by yellow circles. (NASA, ESA, W. Freedman (University of Chicago), ESO, and the Digitized Sky Survey)

Now researchers from Carnegie and the University of Chicago have thrown their hat in the ring. But, rather than remeasuring using existing techniques, the team have calculated the rate of expansion using a new method — one that utilises Red Giants.

Wendy Freedman of the University of Chicago is the lead author of the new study. She explains: “The Hubble constant is the cosmological parameter that sets the absolute scale, size and age of the universe; it is one of the most direct ways we have of quantifying how the universe evolves.”

The team’s research suggests that the discrepancies between the two methods most commonly used may be a result of systematic inaccuracies. That means their new method could resolve the confusion.

Three methods of measuring the Hubble constant

The first method of measuring the Hubble Constant uses Cepheids — stars that pulsate at regular intervals. The rate at which these stars pulsate is directly related to their brightness, meaning that astronomers cans use their luminosity and the period between pulses to measure their distance from Earth with great accuracy.

Carnegie’s Barry Madore, one of the paper’s co-authors, says: Carnegie’s Barry Madore, one of the paper’s co-authors.

“Likewise, comparing how bright distant Cepheids appear to be against the brightness of nearby Cepheids enables us to determine how far away each of the stars’ host galaxies are from Earth.”

When a celestial object’s distance is known, it’s possible to measure the speed at which it is moving away from the Earth by calculating its redshift. This gives astronomers an estimate of the Hubble constant.

The second method — the Planck model — uses the afterglow leftover from an event just after the Big Bang called ‘the last scattering’. This event is the last time in the history of the Universe in which electrons and photons were in thermal equilibrium. It’s also the point that the Universe ceased to be opaque to light — allowing photons to travel unhindered.

This remnant — the cosmic background radiation (CMB) — is the oldest light we can see. It’s also extremely equal across the night sky with a temperature of 2.73K almost everywhere. Patterns of compression in the CMB can still be seen and mapped as slight temperature variations. These ripples, documenting the universe’s first few moments, can be run forward in time through a model and used to predict the present-day Hubble constant.

The former technique says the expansion rate of the universe is 74.0 kilometres per second per megaparsec; the latter says it’s 67.4. If it’s real, the discrepancy could herald new physics.

That’s where our potential third method comes in.

The Carnegie-Chicago Hubble Program, led by Freedman and including Carnegie astronomers Madore, Christopher Burns, Mark Phillips, Jeff Rich, and Mark Seibert — as well as Carnegie-Princeton fellow Rachael Beaton — developed a new way to calculate the Hubble constant.

Their technique is based on a very luminous class of stars called red giants. At a certain point in their lifecycles, the helium in these stars is ignited, and their structures are rearranged by this new source of energy in their cores.

Madore explains: “Just as the cry of a loon is instantly recognizable among bird calls, the peak brightness of a red giant in this state is easily differentiated.

“This makes them excellent standard candles.”

An impressionistic visualization of what’s called the ‘Tip of the Red Giant Branch,’ when diagramming the distribution of stars’ brightness versus their colour. ( Meredith Durbin)

The team made use of the Hubble Space Telescope’s sensitive cameras to search for red giants in nearby galaxies.

Burns continues: “Think of it as scanning a crowd to identify the tallest person — that’s like the brightest red giant experiencing a helium flash.

“If you lived in a world where you knew that the tallest person in any room would be that exact same height — as we assume that the brightest red giant’s peak brightness is the same — you could use that information to tell you how far away the tallest person is from you in any given crowd.”

Once the distances to these newly found red giants are known, the Hubble constant can be calculated with the help of another standard candle — type Ia supernovae — to diminish the uncertainty caused by the red giants’ relative proximity to us and extend our reach out into the more-distant Hubble flow.

According to the red giant method, the universe’s expansion rate is 69.8 — which falls provocatively between the two previously determined numbers.

Stuck in the middle…

Whilst the results do not appear to strongly favour one answer over the other they align more closely with the Planck results the researchers point out.

NASA’s upcoming mission, the Wide-Field Infrared Survey Telescope (WFIRST), scheduled to launch in the mid-2020s, will enable astronomers to better explore the value of the Hubble constant across cosmic time.

WFIRST, with its Hubble-like resolution and 100 times greater view of the sky, will provide a wealth of new Type Ia supernovae, Cepheid variables, and red giant stars to fundamentally improve distance measurements to galaxies near and far.

Madore jokes: “We’re like that old song, ‘Stuck in the Middle with You.

“Is there a crisis in cosmology? We’d hoped to be a tiebreaker, but for now, the answer is: not so fast. The question of whether the standard model of the universe is complete or not remains to be answered.”