The physics world as of today is in chaos. Theories, which were once glorified for their accurate explanations, are now battling the anomalies that are being fired at them by the universe. Be it the fine structure constant or the magnetic dipole moment of electrons, with newer ways of observations comes newer predictions, which demand a drastic change in the existing explanations. This time it is the very famous Hubble constant which is playing its game.
Hubble constant, what is it?
In the1920s, Edwin Hubble, who had just found a new way to estimate the distance of cepheids (special stars whose luminosity rises and falls regularly) from Earth, noticed that everything in the universe moved away from Earth and the speed at which they were moving out increased with their distance. That means that farther away an object is, faster it would be moving. This would mean either of two possibilities – Earth was the centre of the Universe, or the entire Universe was expanding. Hubble theorized that it was the Universe expanding, and gave the very famous constant which still bears his name.
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The constant problem with the constant…
Ever since Hubble gave his constant, it has always been a topic for debate. This is because Hubble has errors in his data and had derived the constant which was approximately 10 times the actual value. And ever since then, new methods have been devised to accurately predict the Hubble constant. And to everyone’s surprise, every method gave entirely a new number. No two methods had matching readings.
One method is to work on the existing datasets on cepheids and accurately calculate the constant. It has been worked up and the value came to be 50,400 mph per million light-years (73.4 km/s/Mpc).
[Image: Space]
Astronomers also devised a new method to determine this constant – the cosmic microwave background, CMB. CMB refers to the radiation emitted during the Bigbang. This has all the information about the initial universe. The Planck satellite of the European Space Agency – ESA has spent 10 years in gathering data about the CMB. The calculated value of the constant from this data comes to be 46,200 mph per million light-years (67.4 km/s/Mpc).
[Image: ESA]
These two numbers may look close, but there is a very huge difference, even after considering their error margins.
If the value calculated from cepheids is correct, there is something entirely wrong with our perception of the universe. This would mean that we need to introduce new and exotic physics into our models of the universe.
If the value from Planck satellite is correct, then astronomers have been always calculating distances in space entirely wrong.
Either of the two possibilities is scary and requires us to change our perception of the universe.
A possible explanation for this anomaly…
There is one more thing this could mean. The universe’s expansion may be accelerated, and the universe is expanding faster now than it was earlier. But this would still require radical transformations of our theories. Many possible explanations have been provided for this anomaly, let’s have a look at few of them.
1.Decaying Dark Matter:
Dark Matter is one of the fundamental ingredients of the universe, but very little is known about it. So the very first thing to get tampered is obviously this. One of the theories proposed says that the dark matter particle decay into a lighter particle and something called the dark photon. This causes the gravitational force exerted by the dark matter to decrease with time and hence accelerating the expansion of the universe.
But the very important drawback of this theory is that for explaining one discrepancy, we are introducing two new unknowns, the lighter dark matter particle and the dark photon. And this is the reason why such a theory makes physicists uncomfortable.
[Image: EarthSky]
2.Variable Dark Energy:
Dark Energy is another mystery which was introduced to resolve the existing mysteries of accelerated expansion of the universe. Theories constructing models for varying dark energy could explain this discrepancy. They do this by stating that the excess of energy present in the early universe provided pressure required to accelerate the expansion. And that is the reason why we see an accelerated expansion today.
But again no perfect model is present for the dark energy, thus any changes would again lead to much more discrepancies.
3.Modified Gravity:
In the standard method, all of the universe’s energy and matter content is fed to Einstein’s equation and the universe’s fate is determined. But some people have tried to modify the recipe itself instead of modifying the contents of the recipe, i.e. tried to modify the theory of gravity itself. William Barker, PhD student at the University of Cambridge believes that a theory of modified gravity could help in explaining this anomaly. The modified gravity could behave as if the universe had excess radiation in the early stage which added to outwards pressure, accelerating the expansion of the universe In the preprint, the authors acknowledge that the model requires more analysis.
This anomaly could be an inadequacy in our observations, or it might be a flaw in our theories, all we can do right now is wait and watch what will be the fate of the universe. But cosmology never stops entertaining us, nobody knows what the universe has in store for us. Anything could be anticipated any day and no surprise is really a surprise because uncertainty is the only certainty in the universe.
