By Larry Klaes
We have much to do as we scramble to explain the universe’s continuing acceleration. Dark energy seems to be demanded by the data, but there are holdouts who argue for a reinterpretation of General Relativity. Tau Zero journalist Larry Klaes looks at one proponent of a revised GR who sees exceptions to the rule in a far earlier era.
Albert Einstein’s work created one of the biggest revolutions in the history of science and radically changed our perceptions of the Cosmos. One of his later breakthrough ideas is the Theory of General Relativity, or GR for short. Really massive objects such as the Sun literally warp space and time around them as they move through the heavens. Since Einstein first published his ideas on GR in 1915, scientists have been able to use the theory to understand the behaviors of even more massive celestial bodies and the very beginning of the Universe itself.
Image: This key Einstein paper included the effect of gravitation on the shape of space and the flow of time, proposing that matter causes space to curve. Credit: NCSA/UIUC.
The Need for a New Look
Now an assistant professor of cosmology at Cornell University thinks she may have found a deviation from the predictions of GR at the very largest cosmological scales. If her finding turns out to be true, it would be a major shift in our understanding of how our Universe works. Rachel Bean, whose paper on the subject has just appeared on the arXiv site, explains that while gravity is very well tested on Solar System scales, when it comes to the distances of the farthest galaxies, which are billions of light years from our own Milky Way galaxy, astronomers still have much to understand.
“Einstein’s theory of General Relativity tells us how gravity determines the relationship between the matter in the Universe and the Universe’s size,” says Bean. “If matter in the Universe is attracted by gravity, then the Universe’s size should be expanding at an ever slowing rate (decelerating). What observations show, though, is that in the last 6 billion years (out of the Universe’s 13.7 billion year existence), the expansion has been speeding up. So we need new physics, labeled ‘dark energy’, to explain this apparent disparity.”
Dark energy is a hypothetical force which makes up almost three-quarters of the mass-energy of the known Cosmos. Cosmologists currently use dark energy to explain why the Universe is expanding when it should otherwise stop moving or even collapse back upon itself.
Scientists do not yet know what dark energy is composed of. Adds Bean:
“There are two broad possibilities for what dark energy is. Either it is a strange, new type of matter that is not attracted by gravity, or Einstein’s Theory of General Relativity needs to be modified on cosmic scales. Part of my research is to try and understand the origins of dark energy and how we can use current and future astrophysical surveys to measure its properties.”
Image: The bending of starlight by matter and energy is well attested. Is there any way General Relativity may have differed in earlier eras? Credit: Ethan Siegel/Lewis & Clark College, OR.
Dark Energy and Cosmic Structure
Bean examined and compared the data collected from the recent Cosmic Evolution Survey, or COSMOS, which involved a number of telescopes both on the ground and in Earth orbit, including the Hubble Space Telescope (HST). The survey examined over two million galaxies, along with distant exploding suns called supernovae and the relic radiation created just 400,000 years after the formation of our Universe in the Big Bang, otherwise known as the Cosmic Microwave Background.
The Cornell scientist says that in order to distinguish between the two possible types of dark energy, astronomers need to include a second type of observations in addition to the distance measurements, namely how galaxies and clusters of galaxies, or large scale structures, evolve over time under the influence of gravity. Bean did this by examining the data on distant “lensed” galaxies whose light from their billions of suns is distorted by other galaxies between themselves and our Milky Way.
“My paper uses a combination of large scale structure and distance measurements to test gravity on cosmic scales to see whether it is consistent with GR or not,” declares Bean. “What I find is that there is a better fit to one time slice in the weak lensing data from the COSMOS survey using the HST, between 3 and 7 billion years after the Big Bang, if gravity is allowed to deviate from GR.”
A Modification to Gravity
If her idea proves to be correct, the results could “indicate that dark energy is related to a modification to gravity rather than a new type of matter.”
While Bean recognizes and admits there may be systematic errors in the data she studied, she also sees her work as a method for using these large scale structure datasets to test theories on dark energy:
“Future astronomical surveys coming in the next six years, namely the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST), of which Cornell has just become an institutional member, will have much improved control over errors in the analysis and will be able to constrain the properties of gravity with far better precision.”
When it comes to the Universe, Bean says she was always interested in astronomy, mathematics, and physics as a child.
“My mum said that I used to go into bookshops even as a small child and buy math books rather than reading books. However, I didn’t get into astrophysics until my mid-20s after a few years working as an analyst at a Management Consultancy. Since then I’ve never looked back….”
The paper is Bean, “A weak lensing detection of a deviation from General Relativity on cosmic scales,” available online.