Since the big bang 13.8 billion years ago, the universe has been expanding. The decisive question is: how fast? This speed is also called the Hubble constant – after Edwin Hubble, who first noticed the expansion of the universe.
Astronomers have come up with two methods to calculate this constant. Method 1 involves the measurements by the ESA’s Planck satellite. This satellite measured the precise structure of the cosmic microwave background, which was produced 380,000 years after the big bang. This can be used to extrapolate the speed of expansion – by using the laws of physics. Calculated in this way, the Hubble constant is 67.0 kilometers per second per megaparsec. This means that for every megaparsec a star is from us, it is moving 67 kilometers per second faster away from us.
The second method for measuring the Hubble constant involves measuring the distances of Cepheids. These are variable stars that pulsate in a certain rhythm. From this rhythm, the true luminosity of the Cepheids can be calculated. So, if we measure their brightness in the sky, we know how far away they are from us. Now we simply need to set the distance in relation to the redshift of the light from the star or the galaxy (redshift is a measure for the expansion rate), and we obtain the Hubble constant.
Researchers have improved this scale so much that the margin of error is now only 2.2 percent. To do this, they combined distance measurement data from ESA’s Gaia space observatory with measurements from NASA’s Hubble telescope, initially for 50 Cepheids. By the start of the 2020s, they want to know the value to one percent precision by incorporating other variables.
The problem: measured using this method, the Hubble constant is 73.5 kilometers per second per megaparsec. That’s more than seven percent different compared with the Planck value. This does not necessarily mean, however, that one of the values is wrong. Planck is measuring the Hubble constant at the dawn of the universe. Hubble and Gaia are taking the pulse of the universe at a much later time. Therefore, it could be that something has changed since that time.
But what could that something be?
- The interaction strength of dark matter might have changed.
- Dark energy might exhibit even stranger behavior than previously thought.
- There might be other, previously unknown particles in the cosmos.
Any of these would mean that physicists would have the long-sought evidence for an expansion of the standard model.
But perhaps the conflict can be solved in some other way. Another research team has proposed a third method for measuring distance – with the help of gravitational waves. In a first measurement using this method with a single source, they determined the Hubble constant with a precision of 14 percent. With additional measurements, however, it should be possible to improve this percentage significantly.