Since the 1990s, thanks to observations by the venerable Hubble Space Telescope (HST), astronomers have contemplated the mystery of cosmic expansion. While scientists have known about this since the late-1920s and early-30s, images acquired by Hubble‘s Ultra Deep Fields campaign revealed that the expansion has been accelerating for the past six billion years! This led scientists to reconsider Einstein’s theory that there is an unknown force in the Universe that “holds back gravity,” which he named the Cosmological Constant. To astronomers and cosmologists today, this force is known as “Dark Energy.”
However, not everyone is sold on the idea of Dark Energy, and some believe that cosmic expansion could mean there is a flaw in our understanding of gravity. In the near future, scientists will benefit from next-generation space telescopes to provide fresh insight into this mysterious force. These include the ESA’s Euclid mission, scheduled for launch this July, and NASA’s Nancy Grace Roman Space Telescope (RST), the direct successor to Hubble that will launch in May 2027. Once operational, these space telescopes will investigate these competing theories to see which holds up.
Not Slowing Down
The expansion of the cosmos was discovered by Belgian astronomer Georges Lemaître in 1927 and independently by Edwin Hubble in 1929. These observations triggered a debate about the nature of the Universe and whether every galaxy emerged from a single event (aka. the Big Bang Theory) or new galaxies were added over time (the Steady State Hypothesis). The debate would be settled with the discovery of the Cosmic Microwave Background (CMB), the “relic radiation” of the Big Bang, and improved instruments that allowed astronomers to look deeper into space (and hence, farther back in time).
Over time, astronomers and cosmologists were able to place tighter constraints on the rate at which the cosmos is expanding – known as the Hubble Constant (or the Hubble-Lemaître Constant). But by the 1990s, observations of Type Ia supernovae (used to measure cosmic distances) revealed that the rate began increasing about 8 billion years after the Big Bang. This contradicted the widely-held idea that cosmic expansion would slow over time as gravity would slowly arrest it, eventually causing the Universe to contract – possibly ending in a “Big Crunch.”
Meanwhile, the rate of expansion came to be known as the Hubble-Lemaître Law (or the Hubble-Lemaître Constant). The fact that it has accelerated over time suggests that something is working against gravity (Dark Energy) or that our understanding of how gravity works on the largest of scales is incomplete. For over a century, scientists have looked to Einstein’s Theory of General Relativity to describe this, but cosmic expansion has led scientists to propose alternate theories – like Modified Newtonian Dynamics (MOND).
Jason Rhodes, a senior research scientist at NASA’s Jet Propulsion Laboratory and a deputy project scientist for Roman, is also the U.S. science lead for Euclid. As he explained in a recent NASA press release:
“Twenty-five years after its discovery, the Universe’s accelerated expansion remains one of the most pressing mysteries in astrophysics. With these upcoming telescopes, we will measure Dark Energy in different ways and with far more precision than previously achievable, opening up a new era of exploration into this mystery.”
Two Observatories
Roman and Euclid will provide separate data streams to fill the gaps in our understanding, hopefully pinning down the cause of cosmic acceleration in the process. This will start with both observatories studying the accumulation of matter using a technique known as “weak gravitational lensing,” where the presence of massive objects in the foreground warps and amplifies light from more distant objects. This phenomenon is predicted by General Relativity, which describes how the curvature of spacetime is altered in the presence of gravitational forces.
In this case, the observatories will look for subtle effects caused by less concentrated masses, like clumps of Dark Matter. This data will be used to make a 3D map of Dark Matter, which is theorized to account for approximately 85% of matter in the known Universe and is what holds galaxies and galaxy clusters together. By mapping the concentrations of Dark Matter, this map will offer clues about the push-pull forces governing our Universe since the gravitational pull of Dark Matter counteracts the expansionary forces of Dark Energy.
The two missions will also study how galaxy clustering has changed from one era to the next. When examining the local Universe, astronomers have noted a pattern in how galaxies are distributed, where any galaxy is twice as likely to have a neighboring galaxy about 500 million light-years away. This distance has grown over time due to the expansion of space, which means that this “preferred distance” has likely changed as well. Seeing how this has varied over time will reveal the expansion history of the cosmos and allow for highly-accurate tests of gravity to see if Dark Energy or MOND is at work.
Roman will also conduct an additional survey of Type Ia supernovae and study how quickly they appear to be moving away from us. Comparing the speed at which they are receding at different distances, scientists will have another means of tracing cosmic expansion and shed light on if and how the influence of Dark Energy has changed over time. They will employ different but complementary strategies to accomplish this and will be much more powerful together than either will be on its own.
Euclid will rely on optical and infrared instruments to observe an area measuring approximately 15,000 square degrees (about one-third) – much larger than the area observed by Roman. It will peer back 10 billion years, roughly 3 billion years after the Big Bang, when the Universe was expanding much slower than it is today. Meanwhile, Roman will study an area measuring about 2,000 square degrees (one-twentieth of the night sky) but in much greater depth and detail. Using its advanced optical and infrared imaging modes, Roman will visualize what the Universe looked like just 2 billion years after the Big Bang.
This will allow Hubble’s successor to examine galaxies that formed during Cosmic Dawn, something the James Webb Space Telescope recently did for the first time. And whereas the Euclid mission will focus exclusively on cosmology, the RST will observe nearby galaxies, stars, and the outer Solar System. These surveys will overlap, allowing scientists to get a “big picture” view of the Universe while simultaneously obtaining highly sensitive and detailed data on individual areas and objects. This will also allow for corrections to be made to Euclid’s surveys, which can be applied to a wider area.
The results will be nothing short of revolutionary, as they will address the most pressing mysteries of modern cosmology and physics. Depending on what they find, Roman and Euclid could confirm that General Relativity and the predominant model of the cosmos – the Lambda Cold Dark Matter (LCDM) model – is correct. On the other hand, they could verify that our models need revision and point the way toward a grand resolution. So it’s either confirmation or resolution. Either way, we can’t lose!
Further Reading: NASA
Euclid is a cool mission, joining similar missions of the US Vera C. Rubin Observatory in Chile 2024 (which it will need to assess galaxy distances), China’s 2-metre Xuntian space telescope to be launched to the Tiangong Space Station 2025, and as mentioned here NASA’s 2.4-metre Nancy Grace Roman Space Telescope to be launched 2027 [Nature].
It was precisely the observational discovery of dark energy that joined the earlier one of dark matter to make a general relativistic universe make sense of star ages et cetera by way of dark energy-cold dark matter (LCDM) theory.
Earlier dark energy missions have been tremendously useful. For instance, the Dark Energy Survey data release 3 joined earlier evidence that the significant differences in various measurements of the Hubble rate is unlikely to change LCDM cosmology. Their cosmological paper show that the cosmological equation of state is robustly observed even so. The state equation – dominated by dark energy and dark matter by the way – express how well the dark energy LCDM model properties predict the observed universe.
People have asked for modifications of general relativity for a century, but observations such as dark matter has only tested it ever better in ever more detail. Meanwhile the improvement on microscale with replacing its nonlinear first order derivative form with the infinite order derivatives of quantum field theory has been performed. [See e.g. effective quantum field theory developer and Nobel Laureate Weinberg’s “On the development of effective field theory” and Donoghue’s “Quantum gravity as a low energy effective field theory” on Scholarpedia.] If we know its effective quantum field theory, what is it observationally about gravity that we don’t know!? Details, at best.
But in any case, the fringe theory which was MOND should perhaps no longer be described as if it was viable in popular descriptions of cosmology. It was developed only to explain spiral galaxy rotation but it – among other similar alternatives – failed when tested on how it behaves when observed by the first multimessenger observation of binary neutron star mergers. [“Troubled Times for Alternatives to Einstein’s Theory of Gravity New observations of extreme astrophysical systems have “brutally and pitilessly murdered” attempts to replace Einstein’s general theory of relativity.” Quanta Magazine, 2018]
By the way, I now realize that the flourish “observationally about gravity that we don’t know!? Details” are important details as going up the energy scale in an effective field theory means having to observe new relevant parameters. But the notion stands.