How big can our universe get?

The universe is expanding. Not only that, the rate at which it is expanding… is expanding! Well, increasing, but that definitely sounded more dramatic. Astronomers have known of this astonishing fact since 1998. But, new data shows that the state of the universe is not quite what we expected.

Around about when any of our fourth-year readers were learning to use the toilet, Earth’s finest astronomers were using NASA’s Hubble Space Telescope to measure the rate of expansion of our universe. They did this using the “cosmic distance ladder”. The closest stars to us can be measured using parallax, seeing how much these stars change their apparent position relative to the very distant background stars, as the Earth orbits the Sun. Then, knowing how far away these stars are, you can calculate how far away more distant stars are by comparing their brightness as if you were stood next to them (hopefully with a bit of sun cream on) to how bright they appear on Earth.

Scientists call these stars “standard candles”.  These brighten and dim reliably according to their ‘true’ intrinsic brightness. Scientists can then use them to ascertain how far away distant galaxies are from us. All astronomers had to do next was calculate the redshift of near, intermediate, and distant galaxies (easy-peasy) to find out how fast these galaxies were travelling away from us, then use the cosmic ladder to calculate how far away they were. Once you’ve got that, you’ve got the rate of expansion of the universe, and you’ve also got Hubble’s constant which tells us how fast a galaxy is moving away from us, relative to how far away it is.

All astronomers had to do next was calculate the redshift of near, intermediate, and distant galaxies (easy-peasy)

The accuracy of the Hubble constant depends heavily on the accuracy of the cosmic ladder. Recently, astronomers have been working on reinforcing and generally improving the cosmic ladder. New data comes from measuring the parallax of eight stars 10 times further away than the ones considered previously.

If you remember, parallax was the method used to gauge the distance of the nearest stars, the bottom rung on the cosmic ladder. If we can get our first rung secure, we’re much less likely to fall off in our future measurements. These stars pulsate at longer time intervals as well, decreasing any percentage errors.

Recently, astronomers have been working on reinforcing and generally improving the cosmic ladder

However, due to their distance, the apparent changes in position are just 1/100th the size of one of Hubble’s pixels. About the size of a grain of sand 100 miles away. Eek. Scientists being as ingenious as they are, however, developed a scanning technique, capturing the stars’ positions 1000 times a minute, every six months for four years. With the bottom rung now well and truly holding their footing, they were able to calculate the distances to far away galaxies to a much greater precision. What they found was just as exciting as their brand-new ladder, that the value for the Hubble constant… does not agree with what we expect from our observations of the early universe.

ESA’s Planck satellite, which maps the cosmic microwave background radiation, the fossil of the early universe, says the Hubble constant should be 67 km per second per megaparsec (where one megaparsec is equal to 3.3 million light-years). This means that for every 3.3 million light-years that a galaxy is from Earth, it is moving away from us 67 km per second faster. It also implies that a galaxy 4,478 megaparsecs away is moving away from us faster than the speed of light (don’t worry this doesn’t break physics, nothing can travel faster than light through space, the space itself is expanding).

It also implies that a galaxy 4,478 megaparsecs away is moving away from us faster than the speed of light

Nonetheless, the recent experiment has put the Hubble constant at 73 km per second per megaparsec. Due to the increased accuracy in the cosmic ladder, the chances that this difference in Hubble and Planck is a coincidence is about one in 5000. What could be driving this expansion? Matter attracts matter, what goes up must come down, surely gravity would ensure that everything doesn’t get too far apart?

If they haven’t yet, this is where things will start to get weird. And dark. Very dark. The accelerating expansion of the universe is believed to be caused by a very mysterious substance that makes up 95% of the universe. This is what scientists call “dark energy”. It is believed to have a constant volume density in space, meaning as space gets bigger, there’s more dark energy present, driving the galaxies further and further apart. Energy conservation is for a static universe, but we’re expanding. The question we’ve got to ask is, will this universe expand forever?

Due to the increased accuracy in the cosmic ladder, the chances that this difference in Hubble and Planck is a coincidence is about one in 5000

It’s a battle between the resistive push of dark energy versus the attractive pull of gravity. Nevertheless, with the value of the Hubble constant higher than we would expect from early models, it is near certain that this expansion will continue until the universe is so vast and cold that there will be no galaxies, no stars, no life. So I for one am certainly glad I can still take an evening walk and look up at the stars, the nebulae and the supernovae before they all run away.