26 April 2019 News

As mystery of the Universe’s expansion rate widens, a simple solution is offered

An image of the Large Magellanic Cloud, a satellite galaxy of the Milky Way located nearly 200 000 light-years from Earth. Image: NASA, ESA. Acknowledgement: Josh Lake
An image of the Large Magellanic Cloud, a satellite galaxy of the Milky Way located nearly 200 000 light-years from Earth. Image: NASA, ESA. Acknowledgement: Josh Lake

There is something amiss with the expansion of the Universe; the space between galaxies is stretching – scientists are sure about that – but just how fast is it expanding? New research shows that what scientists predict and what they observe are two different things and measurements calculated of today’s expansion rate do not match the rate that was expected based on how the Universe appeared shortly after the Big Bang over 13 billion years ago.

The unit of measurement used to describe the expansion of the Universe is called the Hubble Constant, H0, (see notes below*) and the current best direct measurement of the Hubble constant is 74.03 kilometres per second per megaparsec (km/sec/Mpc) – give or take 2.4 km/sec/Mpc, including both random and systematic errors. This means that for every 3.3 million light-years farther away a galaxy is from us, it appears to be moving 74 kilometres (46 miles) per second faster, as a result of the expansion of the Universe.

Scientists have previously worked out this value by measuring the light from pulsating stars called Cepheid variables in a neighbouring satellite galaxy known as the Large Magellanic Cloud (LMC). Cepheids vary their brightness in such a dependable and regular way that they have become an important “cosmic distance ladder" to aid scientists determine how far away things are in the Universe.

Indeed, this new research uses the same type of object but utilises a different method to calculate the Hubble Constant. Instead of observing one Cepheid at a time with NASA’s Hubble Space Telescope as it makes its 90-minute orbit around Earth, a team of scientists including Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, in Baltimore, Maryland has used Hubble as a "point-and-shoot" camera to snap quick images of the extremely bright pulsating stars,

This method means that Hubble’s telescope pointing doesn’t need to be constantly recalibrated and this quick fire method has earned it the title DASH, which stands for Drift And Shift. “These Cepheids are so bright, we only need to observe them for two seconds. This technique is allowing us to observe a dozen Cepheids for the duration of one orbit. So, we stay on gyroscope control and keep 'DASHing' around very fast,” says Reiss. The improved method has now given scientists the most precise value yet of the Hubble Constant.

If it all sounds great, it is. No really it is. But herein lies a spanner in the works; the method outlined above and the value it produced are not the only one.

Looking at this phenomena from a different angle is the European Space Agency’s Planck mission. Planck has been studying the skies since its launch nearly a decade ago to refine values associated with the cosmic microwave background (CMB). The CMB is relic radiation leftover from the Big Bang and as such gives scientists a look into conditions in the very young Universe.

However, when you calculate the Hubble Constant from CMB/Planck data, you get a different figure altogether; 67.4 km/sec/Mpc, with a tiny uncertainty of less than a percent.

This latest research by Reiss and team has reduced the uncertainty in their Hubble constant value to an unprecedented 1.9 percent - that is a significant improvement from a previous estimate last year which had the uncertainty set at 2.4 percent. This is equivalent to saying that the discrepancy is only a fluke to 1 in 100,000 (last years estimate was a chance of 1 in 3,000, so a substantial gain from earlier data).

In principle these two speed values should agree to within their respective uncertainties, but they don’t and this discrepancy is a lot by scientific standards. Both methods have been thoroughly vetted and no one is quite sure whether differences in measurement techniques are to blame, or whether results from unlucky measurements are the cause. Like many things in life that don’t agree, this discrepancy has caused what cosmologists call a 'tension' – an oddity that still needs explaining.

But says Reiss, this disparity could not plausibly occur just by chance. "This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This is not just two experiments disagreeing. We are measuring something fundamentally different. One is a measurement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding,” explained Reiss.

“If these values don't agree, there becomes a very strong likelihood that we're missing something in the cosmological model that connects the two eras,” he said.

So, what could it be that is missing? One explanation proposed by astronomers at Johns Hopkins, is a theory dubbed "early dark energy," and suggests an unexpected appearance of dark energy in the young Universe. This would require dark energy, which is thought to now comprise 70 percent of the Universe's contents, appeared not long after the big bang, that made the Universe expand faster than astronomers had predicted. The existence of this "early dark energy" could account for the tension between the two Hubble constant values, Riess said.

Do we really need to add another tier of exotic dark stuff to explain the inconsistency in results? So much uncertainly surrounds the original invisible repulsive force that was first postulated to account for the expansion without adding a third layer to it all.

There are others that think such dark energy ideas are now getting too convoluted, and that a much simpler explanation, one that even Einstein considered, should now be given serious consideration; a change in the speed of light, or VSL (Varying Speed of Light) as others know it by.

“The ‘tension’ between measurements of the Hubble-Lemaire constant, H0, (which is known to be changing over time) shows that old theories of the Universe are missing something. If H0 was the lower value of 67 km/sec/Mpc, much or all of the so-called acceleration would vanish. The differing values may be explained if the speed of light has changed between the early and late universe,” said Louise Riofrio, an author and scientist who now works at an observatory association in Hawaii.

The theory that light has slowed down ever so slightly year after year since the Big Bang occurred has perplexingly for those who advocate this idea, remained out of mainstream physics. But like Riofrio, there are others who postulate that a changing speed of light can not only account for a simplified explanation of an expanding Universe without invoking the need for dark energy, it can explain many other cosmological phenomena too.

This basic but powerful enough idea, which should not be confused with faster than light theories, could soon be tested with the help of ESA’s Atomic Clock Ensemble in Space (ACES) mission which is scheduled to fly to the International Space Station next year. Riofrio has written an exclusive article for ROOM magazine explaining the theory behind a change in light speed that will be published in our Summer issue.

[Notes*: The unit of measurement used to describe the expansion of the universe is called the Hubble Constant, H0, and is named after Edwin Hubble, an astronomer who after systematically measuring the distances to far-off galaxies, observed that the red shift of galaxies was directly proportional to the distance of the galaxy from earth. Red shift is the stretching of a light’s wavelength, so that the light is seen as 'shifted' towards the red part of the spectrum. This was interpreted that things farther away from Earth were moving away faster.

This concept lies at the heart of our modern cosmology, in which the entire Universe and all that fills it such as space, time and matter, burst into existence in a Big Bang.

Hubble's initial value for the expansion rate was approximately 500 km/sec/Mpc or about 160 km/sec per million-light-years. This rate though inferred that the Universe would only be 2 billion years old! Since then, many new techniques have been developed and refined in order to arrive at the answer that is calculated today.

Hubble was not the first person to realise this and indeed the International Astronomical Union – the same organisation that demoted Pluto to dwarf planet status – voted to rename Hubble’s law as the Hubble–Lemaître law, in honour of Belgian priest and astronomer Georges Lemaître, who described how the expansion of the Universe would cause galaxies to move away from Earth at speeds proportional to their distance some two years before Edwin Hubble did.]

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