An artist’s impression of how ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun, may have looked. Credit: ESO/M. Kornmesser, CC BY 4.0
Bengaluru: For over a century, cosmologists have assumed that the universe is homogenous and appears the same in all directions. But in the last few years, many physicists and astronomers have challenged this so-called cosmological principle.
A group of scientists from the US, the UK, France and India have gathered more evidence that suggests the universe isn’t in fact the same in all directions.
Their paper was recently published in the Astrophysical Journal Letters.
“We are suggesting that the universe does not look the same in all directions – it is not isotropic,” Mohamed Rameez, a reader of high-energy physics at the Tata Institute of Fundamental Research, Mumbai, and one of the paper’s authors, said.
In the late 1920s, Edwin Hubble made a groundbreaking discovery that confirmed that the universe was expanding: seen through telescopes, galaxies seemed to be moving away from each other.
Nearly seven decades later, two research groups independently reported that our universe was expanding at an accelerating pace. The groups, each led by Saul Perlmutter and Brian Schmidt, observed distant type 1a supernovae – massive stars that had run out of fuel and exploded, with a fixed brightness. By studying the light from these objects, the cosmologists estimated how far away they were in space (and therefore earlier in time).
Combining this information with the redshift – the change in wavelength of the stretched light, due to the Doppler effect – they worked out how fast the universe was expanding.
While doing this exercise, the teams found something unexpected: the light emitted from these supernovae was dimmer than they expected.
The best explanation for this was that the universe was expanding faster than they anticipated.
Perlmutter, Schmidt and their teams suggested that a mysterious form of energy, called dark energy, was driving this accelerated expansion of our universe.
Imagine you’re driving your car down the road and you suddenly turn off your engine. The car keeps moving for some distance and then stops. The universe was thought to be like this. Shortly after being created in the Big Bang, it underwent a period of rapid expansion, called cosmic inflation (it’s a popular theory but we’re yet to find direct evidence of it.)
This inflation is the engine. Since the inflationary epoch is over, the universe’s expansion should be slowing down, more so since gravity has been applying the brakes by pulling matter together.. But if the universe is expanding faster even today – if your car is accelerating a long time after the engine has been turned off – something must be pushing on it. Astrophysicists call this entity dark energy.
Today, almost everyone knows that the answer to the question ‘what is the universe made of?’ goes something like this: About 68% of all the energy in our universe is dark energy, about 27% is dark matter[footnote]By the mass-energy equivalence[/footnote], and less than 5% constitutes all normal matter.
Scientists and artists alike have contemplated the implications of a universe expanding faster and faster in many papers and projects. A 2016 study estimated that the universe is not only expanding – but is doing so 5-9% faster than we figured. In his 2007 short story, Last Contact, Stephen Baxter chronicled the last moments of life in a town as dark energy rips the universe apart.
Perlmutter and Schmidt’s work even acquired a sense of finality – of the case being closed, so to speak – when they were awarded the Nobel Prize for physics in 2011, along with Adam Riess. The citation read: “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae”.
But for all this memorialisation, the case of dark energy driving the universe’s accelerated expansion may not yet be closed.
The principle
Perlmutter, Schmidt & co.’s important discovery hinged on the cosmological principle, which assumes that the universe is isotropic: the velocity or the number of objects in different directions must be the same (across very large distances).
If the cosmological principle wasn’t true, the implication would be that it’s okay for radiation in one direction to be dimmer than in the other, eliminating the need for the universe to be expanding at an accelerated pace to explain what Perlmutter, Schmidt and their groups found.
The new study disputes the truth of the cosmological principle.
The very first neutral hydrogen atoms created after the Big Bang emitted some radiation that flows through the universe even today. It’s called the cosmic microwave background (CMB). The CMB doesn’t seem to be isotropic, however: it is hotter in one direction in the sky and colder in the other, by a 1,000-times.
Cosmologists assume that this ‘dipole anisotropy’ arises because Earth is moving at 369 km/s with respect to a reference frame in which the universe is isotropic. They believe that the resulting Doppler shifts in their observations can explain this odd feature.
In the current study, the researchers have rejected this interpretation at a statistical significance of 4.9 sigma. This means the chance that the result is an accident is only 1 in 2 million.
“This suggests that the rest frame of matter and radiation do not coincide in the universe even on very large scales – hundreds of millions of light years or more,” Rameez said. According to him, their finding has profound implications for cosmological observations that imply the universe is dominated by dark energy.
Pankaj Jain, a professor of physics at IIT Kanpur, said, “The study suggests that the universe may not be isotropic and hence the standard Big Bang paradigm, which assumes isotropy, may not be valid.”
Prasun Dutta, an assistant professor of physics at IIT-BHU Varanasi, agreed. “This result suggests that we need to reexamine the cosmological principle first observationally and maybe with more precise and different probes,” he said. “Taking cues from these observations, we may then have to reformulate or modify the cosmology we know today.”
Jain also noted that this study follows up on a large amount of work by many scientists in the late 1990s. Ashok Singal, a physicist at the Physical Research Laboratory, Ahmedabad, made one of the earliest claims that the CMB is not isotropic – even after factoring in Earth’s relative motion.
Jain and his colleagues also started working on the problem around 1998. They found, among other things, that radio waves were polarised relative to the Milky way’s axis to different extents in different directions.
“At the time, very few scientists would take this seriously. But now a large body of work is devoted to the study of this effect, both theoretically and observationally,” Jain said. “If this turns out to be true, we will need to evaluate the isotropy of all cosmological observables by using dedicated surveys.”
As a graduate student in particle physics, Rameez was looking for signs of dark matter using the IceCube neutrino telescope at the South Pole. When he couldn’t find these signs, he started thinking about what the negative result could mean for the idea of dark matter.
He subsequently joined Subir Sarkar at Copenhagen, where he also met Roya Mohayaee and Jacques Colin – all authors of the new paper. They had found that very distant radio galaxies exhibit an unusual dipole anisotropy, the first hints of which Singal had noticed.
Around this time, the Perlmutter and Schmidt groups published the raw data of the type Ia supernovae they had studied. Subir Sarkar’s group then set about assessing how robust the finding was, through rigorous statistical analysis of the procedure that the researchers had used to adjust the brightness of the 740 supernovae. Sarkar’s group found that the evidence for the accelerated expansion of our universe was marginal.
Members of the team also analysed a giant database of quasars – extremely bright galactic centres with black holes at the centre – that Nathan Secrest, of the US Naval Observatory in Washington, had put together in 2015. They found that the dipole they observed with the quasars was in the same direction as the dipole of the CMB. However, the difference was much higher – almost double the expected value, a strong signal that the universe is not isotropic.
“The idea that the universe is anisotropic is being increasingly accepted as an empirical fact as there are too many datasets showing it now to deny it,” Rameez said.
But despite the strength of their observations, Rameez said, “we really have to be sure that we are not misinterpreting what we are seeing.” The cosmological principle is a longstanding tenet of studies of the universe, and contradicting it will need extraordinary evidence.
There’s also the fact that the existing model of the universe, called the lambda CDM model, explains so much else about the universe to scientists’ satisfaction. It has been studied, tested and refined over decades – and it can’t simply be wrong.
The lambda CDM contains parameters that account for the universe’s composition. The Greek letter lambda (λ) denotes the cosmological constant, representing the density of dark energy. CDM stands for the kind of dark matter that only absorbs light and does not emit it, and which astrophysicists think wasn’t moving when the expansion of the universe sped up.
Perlmutter, Schmidt and others later invoked the cosmological constant, a number that accounts for the accelerated expansion of the universe.
The lambda CDM model
In short, the new study asks if the value of the cosmological constant is correct.
But many cosmologists and astrophysicists still hold that the current model best explains our universe.
“Inhomogeneities could change lambda by 1 or 2%, but could not get rid of it. It’s simply impossible,” Ruth Durrer, a professor of astroparticle physics at the University of Geneva, told Quanta in 2019.
Jasjeet Singh Bagla, a professor of physical sciences at IISER Mohali, also agreed that the lambda CDM model is the best we have. “It is consistent with the bulk of the observational data we have,” explained. “There are specific areas with differences between predictions and observations. But people are working on these aspects to see whether a more nuanced estimate is required or some new physics needs to be added.”
According to both Rameez and Jain, they are making every effort to look for alternate explanations that could undermine their result – and haven’t found any major concerns so far. “But we are keeping our eyes open,” Rameez said.
Jain said that scientists must carefully account for bias in data before drawing conclusions. According to him, cosmological surveys are potentially affected by systematic biases that have to be corrected.
Bagla said that the authors hadn’t said why they chose to work with the number of sources and not the fluxes – the energy emitted per unit area that scientists typically use – to understand why the properties of quasars were uneven in different directions.
He added that it would be interesting to see what happens if one chose to calculate the dipole with the fluxes and other methods that the paper notes as an aside.
Sebastian von Hausegger, a postdoctoral researcher at Oxford and another author of the new study, is leading a follow-up looking at multiple quasar catalogues. The researchers are also planning to study the sources of radio and infrared waves in the universe to refine their results.
In the long run, they are curious to see what a cosmological model that explains the universe’s anisotropy will look like.
Joel P. Joseph is a science writer.