The universe is expanding. but astrophysicists do not know how quickly.

Hubble captured this image of the universe, many galaxies, with an Einstein-ring to boot. If the light from distant galaxies distortion around an extremely large mass, like a cluster, it creates this elegant ring. Credit: ESA/Hubble and NASA; Acknowledgement: Judy Schmidt

The next time you eat a blueberry (or chocolate chip muffin consider what happened with the blueberries in the batter as it was fried. The blueberries started all squeezed together, but if the muffin extensive they began to move away from each other. If you could sit on a blueberry you would see that all the others of you, but the same would apply for the blueberry you have selected. In this sense, galaxies are a lot like blueberries.

Since the Big Bang, the universe has been expanding. The strange thing is that there is no single place that the universe is expanding, but all galaxies (on average) of all the others. From our perspective in the Milky way, it seems like most of the galaxies away from us – if we are the center of our muffin-like universe. But it would be exactly the same from another galaxy – everything is moving away from the rest.

To make matters even more confusing, new observations suggest that the rate of this expansion in the universe may be different, depending on how far you look back in time. These new data, published in the Astrophysical Journal, indicates that it might be time for a revision of our understanding of the cosmos.

Hubble’s challenge

Cosmologists characterize the universe, which the expansion in a simple law known as the Law of Hubble (with the name of Edwin Hubble – although, in fact, many other people broken down Hubble’s discovery). Hubble’s Law is the observation that more distant galaxies are at a faster pace. This means that galaxies that are close to relatively slowly in comparison.

The relationship between the speed and the distance of a galaxy is set by the “Hubble Constant,” which is about 44 miles (70km) per second per Mega Parsec (a unit of length in astronomy). What this means is that a galaxy gains about 50,000 km per hour for every million light-years away from us. In the time that it takes to read this sentence, a galaxy at a million light-years away is moving by about an additional 100 km.

This expansion of the universe with galaxies moving away slower than distant galaxies, is what one would expect for a uniformly expanding universe with dark energy (an invisible force that causes the universe’s expansion to accelerate ) and dark matter (an unknown and invisible form of matter, that is five times more than normal matter). This is what one would also observe of blueberries in a growing muffin.

The history of the measurement of the Hubble Constant is fraught with difficulties and unexpected revelations. In 1929, Hubble himself thought the value should be about 342,000 km per hour, per million light-years – about ten times larger than what we measure now. Accurate measurements of the Hubble-telescope is Constant over the years is actually what led to the unintended discovery of dark energy. The search to find out more about this mysterious form of energy, that 70% of the energy of the universe, has inspired the launch of the world (at this moment) best space telescope, named after the Hubble telescope.

Cosmic eye-catcher

Now it seems that this problem can be continued as a result of two extremely precise measurements that do not agree with each other. As cosmological measurements became so precise that the value of the Hubble constant was expected to be for once and for all, it is found in place of that things don’t make sense. Instead of one now we have two showstopping results.

On the one side we have the new, very precise measurements of the Cosmic Microwave Background – the afterglow of the big Bang – the Planck mission, which measured the Hubble Constant to be about 46,200 miles per hour per million light years (or with the help of cosmologists’ units 67.4 km/s/Mpc).

On the other hand, we have new measurements of pulsating stars in local galaxies, also very accurate, which is measured by the Hubble Constant to be 50,400 km-per-hour, per million light years (or with the help of cosmologists units 73.4 km/s/Mpc). This is closer to us in time.

Both these measurements claim that their result is correct and accurate. The measurements’ uncertainties are only about 300 miles per hour, per million light-years away, so it really seems like there is a significant difference in movement. Cosmologists refer to this disagreement about the “tension” between the two measurements – they are both statistically draw results in different directions, and there is something of a snap.

New physics?

So what goes snap? At the moment the jury is out. It is possible that our cosmological model is wrong. What is seen is that the universe is expanding faster than we would expect on the basis of the more remote measurements. The Cosmic Microwave Background measurements, not the measurement of the local extension directly, but are derived from these via a model our cosmological model. This has been immensely successful in predicting and describing many observational data in the universe.

So while this model might be wrong, no one is to come up with a simple, convincing model that can explain this and, at the same time, explain everything what we perceive. For example, We can try and explain this with a new theory of gravity, but other observations do not fit. Or we can try and explain a new theory of dark matter or dark energy, but further observations do not fit, and so on. So if the voltage is due to the new physics, it should be complex and unknown.

A less exciting explanation could be that there are “unknown unknowns” in the data caused by systematic effects, and that a more careful analysis might one day reveal a subtle effect that has been overlooked. Or it could just be statistical fluke that will disappear as more data are collected.

It is currently unclear what the combination of the new physics, systematic effects or new data will resolve this tension, but there is something to give. The extension of muffin picture of the universe can no longer work, and cosmologists are in a race to win a “great cosmic bake-off” to explain this result. If new physics is required to explain these new measurements, then the result will be a showstopping the change of our image of the cosmos.

Thomas Kitching, Reader in Astrophysics, UCL

This article was originally published on The Conversation. Read the original article.


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