We can add yet another way to measure the universe’s expansion onto the pile of controversy that could perhaps be the most exciting story in cosmology today.
The universe is expanding. Measurements of the most distant detectable electromagnetic radiation predict one value for the rate of expansion, but measurements gleaned from nearer objects reveal different values.
If the values really are incompatible, it could be a sign that the grand theory currently used to describe the universe’s evolution is broken. Now, a team of scientists is excited about a new method they’ve devised, which relies less on human assumptions about how the universe works.
“Our method is insensitive to the choice of cosmological model,” Inh Jee, the study’s first author from the Max Planck Institute for Astrophysics, told Gizmodo. “That’s what we really want to emphasise.”
Edwin Hubble convinced astronomers in the 1920s that distant objects were moving away from us, but the rate they were receding, called the Hubble constant, has been a subject of debate ever since.
New telescopes have led to new observations, like those taken by the Planck satellite which have determined that the Hubble constant equals 67.4 kilometres per second per megaparsec — meaning that for every 3.26 million light-years in distance (called a megaparsec), objects appear to be moving away from each other another 67.4 kilometres per hour. But observations based on a slew of properties from more nearby light sources have revealed other values for the rate of expansion, always larger.
Physicists now debate whether these values are actually larger, and if so, whether it’s due to something about the way they calculate the distances to these objects or if it’s truly a sign of undiscovered physics. A recent, excellent article in Quanta Magazine summarises the story and its drama.
The problem mostly hinges on the difficulty of measuring the distance to things. Scientists typically rely on objects with a known brightness, called standard candles — brighter, and they’re closer and dimmer, they’re farther away. Such objects include certain supernovae and stars that flicker at a rate dependent on their brightness. Scientists might also rely on so-called standard rulers, objects whose size is known and whose distance can be calculated based on how big or small they appear in the sky.
One of the teams measuring the Hubble constant, called H0LiCOW (pronounced the way it looks), was using one of those ruler methods to determine distances, and have now improved it such that it relies less on human assumptions, Jee explained.
This method calculates the radius of a distant object (called a gravitational lens) and uses it as a ruler; that ruler can then help provide an accurate absolute distance to standard-candle supernovae, according to the paper published in Science.
When looking at a massive object like a galaxy, you will see around it multiple images of the bright objects behind it, because its huge gravity warps light like a lens (hence the name “gravitational lens”). Sometimes the background objects even appear warped into a ring. If a background object is flickering, then each image of it might flicker at different times, based on the distance the bent light travels.
Scientists can also measure the velocity of stars orbiting in these distant galaxies, which reveals the galaxy’s gravitational potential and mass. Combining this information lets them calculate the distance to the lensing galaxy and its size.
The researchers can then use the lensing galaxy as a standard ruler and a calibrator to calculate the absolute distance to certain supernovae (the ones that are traditionally used to determine the Hubble constant). This allows them to calculate the constant in a way that relies less on human assumptions about things like how much dark matter and dark energy exists in the universe.
With their new method, the researchers calculated a value for the expansion of the universe based on just two objects. They came up with a very high 82.4 kilometres per second per megaparsec, but with statistical error bars so large that it’s not really worth considering yet. After all, this study is just a pilot.
Adam Reiss, Johns Hopkins University astronomer and leader of a Hubble constant-measuring team called SH0ES, told Gizmodo in an email that the result isn’t conclusive, but, “It’s nice to see people look for alternative methods, so props for that.”
And removing dependence on assumptions is “important when attempting to pinpoint the source of discrepancies between different techniques,” Tamara Davis, Australian astrophysicist at the University of Queensland, wrote in an accompanying commentary for Science.
There are lots of different methods in the works for determining the Hubble constant, be it using standard candles and rulers or even using colliding neutron stars (though we actually need to detect more colliding neutron stars for that method to work).
The H0LiCOW team plans reduce their experimental error soon and use their method to determine distances to stars by measuring more lenses and the motion of the stars inside the lenses, Jee told Gizmodo.
The Hubble constant discussion will continue to be of importance to physicists, because it represents a place where our most successful theory of the universe breaks down — a place for new ideas and new experiments to reveal how the cosmos really works.