How Big Our Universe

 

The cosmos has been expanding since the Big Bang, but how fast? The answer could reveal whether everything we thought we knew about physics is wrong.

Let’s start by saying the Universe is big. When we look in any direction, the furthest visible regions of the Universe are estimated to be around 46 billion light years away. That’s a diameter of 540 sextillion (or 54 followed by 22 zeros) miles. But this is really just our best guess – nobody knows exactly how big the Universe really is.

That is because we can only see as far as light (or more accurately the microwave radiation thrown out from the Big Bang) has travelled since the Universe began. Since the Universe burst into existence an estimated 13.8 billion years ago, it has been expanding outwards ever since. But because we don’t know a precise age for the Universe either, it makes it tricky to pin down how far it extends beyond the limits of what we can see.

If the Universe is really expanding faster than we thought, it might be much younger than the currently accepted 13.8 billion years

Freedman and her team were the first to use Cepheid variables in neighbouring galaxies to our own to measure the Hubble constant using data from the Hubble Space Telescope. In 2001, they measured it at 72km (45 miles)/s/Mpc.

Since then, the value from studying local galaxies has hovered around the same point. Using the same type of stars, another team used the Hubble Space Telescope in 2019 to arrive at a figure of 74km (46 miles)/s/Mpc. Then just a few months later, another group of astrophysicists used a different technique involving the light coming from quasars to get a value of 73km (45 miles)/s/Mpc.

If these measurements are correct, then it suggests that the Universe might be inflating faster than theories under the Standard Model of Cosmology allow. It could mean this model – and with it our best attempt at describing the fundamental nature of the Universe – needs to be updated. At present, the answer is not certain, but if it proves to be the case, then the implications could be profound.

“It could be telling us something is missing from what we think is our standard model,” says Freedman. “We don’t yet know the reason why this is happening, but it’s an opportunity for a discovery.”

If the Standard Model is wrong, one thing it could mean is our models of what the Universe is made up of, the relative amounts of baryonic or “normal” matter, dark matter, dark energy and radiation, are not quite right. And if the Universe is really expanding faster than we thought, it might be much younger than the currently accepted 13.8 billion years.

An alternative explanation for the discrepancy is the part of the Universe we live in is somehow different or special compared to the rest of the Universe, and that difference is distorting the measurements. “It is far from a perfect analogy, but you can think about how the speed or acceleration of your car is modified if you go up or down a hill even if you are applying the same pressure to the gas pedal,” says Beaton. “I think it is unlikely to be the ultimate cause of the discrepancy in the Hubble constant that we see, but I also think that it is important to not disregard the work put into those results.”

But astronomers think they are getting close to pinpointing what the Hubble Constant is and which of the measurements is correct.

“What’s exciting is I think we really will resolve this in fairly short order, whether it’s a year or two or three,” says Freedman. “There are so many things that are coming on the horizon that will improve the accuracy with which we can make these measurements that I think we will get to the bottom of this.”

One is the ESA’s space observatory Gaia, which launched in 2013 and has been measuring the positions of around one billion stars to a high degree of accuracy. Scientists are using this to work out the distances to the stars with a technique called parallax. As Gaia orbits the sun its vantage point in space changes, much like if you close one eye and look at an object, then look with the other eye it appears in a slightly different place. So, by studying objects at different times of the year during its orbit, Gaia will enable scientists to accurately work how fast stars are moving away from our own Solar System.

Another facility that will help answer the question of what the Hubble Constant’s value is the James Webb Space Telescope, which is due to be launched late in 2021. By studying infrared wavelengths, it will allow better measurements that won’t be obscured by the dust between us and the stars.

Think you see a pearl dangling lustrously in Vermeer’s famous portrait of a girl endlessly turning towards or away from us? Think again. The swollen bauble around which the painting’s mystery spins is just a pigment of your imagination. With a flick of the wrist and two deft dabs of white paint, the artist has tricked the primary visual cortices of our brains’ occipital lobes into magicking a pearl from the thinnest of air. Squint as tight as you wish and there is no loop that links the ornament to her ear. Its very sphericity is a hoax. We’ve willed the earring into weightless suspension from the puniest of white apostrophes. Vermeer’s precious gem is an opulent optical illusion, one that reflects back on our own illusory presence in the world.

If they find that the difference in the Hubble Constant does persist, however, then it will be time for new physics. And although many theories have been offered up to explain the difference, nothing quite fits what we see around us. Each potential theory has a downside. For example, it might be there was another kind of radiation in the early universe, but we have measured the CMB so accurately this does not seem likely. Another option is that dark energy could be changing with time.

“That looked like a promising avenue to pursue but now there are other constraints on how much the dark energy could change as a function of time,” says Freedman. “You’d have to do it in a really contrived way and that doesn’t look very promising.” An alternative is that there was dark energy present in the early universe that just disappeared, but there is no obvious reason why it would do this.

It has forced scientists to dream up new ideas that could explain what is going on. “People are working really hard at it and it’s exciting,” adds Freedman. “Just because no one’s realised what [the explanation] is yet doesn’t mean that there won’t be a good idea that will emerge.”

Depending on what these new telescopes reveal, Beaton and Freedman could well find themselves in the midst of a mystery worthy of an Agatha Christie novel after all.