‘We are seeing a sign that dark energy is not a cosmological constant’

Image: Custom colormap package by cmastro; Claire Lamman / DESI collaboration

On April 4, 2024, the Dark Energy Spectroscopic Instrument (DESI), a collaboration of more than 900 researchers from over 70 institutions around the world, announced that they have made the most precise measurement of the expansion of the universe and its acceleration. 

The team found that the universe is expanding at a rate of 68.5 (±0.6) kilometres per second per megaparsec (a million parsec; 1 parsec equals 3.2616 light years) by analysing results collected from year one of the survey. The expansion slightly increased in strength around 5 billion years ago, a little before the Earth formed at around 4.53 and 4.58 billion years ago.

Their results are largely consistent with the cosmological model, which describes our understanding of the Universe and its contents. Interpretation of observational data in the framework of this model has led to the conclusion that about 70 per cent of the Universe is in the form of a mysterious ‘dark energy’, which is causing its expansion rate to accelerate, according to the University of Oxford. The team, however, found some subtle deviations to the standard model, which, if proven, could bring in new ideas to explain the universe.

Down To Earth spoke with Shadab Alam, faculty at the Department of Theoretical Physics of the Tata Institute of Fundamental Research, Mumbai, to understand the significance of the new findings. 

Rohini Krishnamurthy: The DESI group has unveiled the largest and the most detailed 3D map of the universe’s expansion over the years. Could you describe what the map looks like and how it captures the distribution and movement of galaxies with time?

 

 Shadab Alam: The DESI survey figures out the distances of cosmic objects by measuring what we call ‘light fingerprint’ coming from this galaxy. So, for each of these galaxies, we have their fingerprint, which is called ‘spectra’ in technical terms [spectra is a graph that shows the intensity of light being emitted over a range of energies. The spectrum of a galaxy contains contributions from stars, gas and (sometimes) dust].

Once you measure the galaxy spectra, you are able to tell a lot about these galaxies and the universe, which is a collection of different kinds of galaxies, including supermassive blackholes. 

We have collected a lot of detailed information for more than six million galaxies. For each galaxy in our map, we have locations on the sky as well as their fingerprint and from the fingerprint, we can glean lots of information about them, including their mass, distance, host blackhole mass inside them and the like.

RK: The DESI group has measured the universe’s expansion rate or Hubble constant at 68.5 (±0.6) km / sec per megaparsec. What does this mean?

SA: The universe is expanding but the rate increases as you move away to larger and larger distances. The hubble constant is essentially quantifying the rate, and how it changes as a function of distance.

If we have two imaginary points in free space separated by the distance between the Sun and Earth, then they we will move away with a speed such that it will take 14,500 years (+/- 200 years) to travel from Mumbai to Pune (about 150 km apart). 

What I have essentially done is to multiply the Hubble’s constant with the Earth-Sun distance and then divide the distance (150 km) by that number.

Another interesting reason we are studying the expansion of the universe is because we want to really understand Hubble constant and how it changes with time. The Hubble constant now [68.5 (±0.6) km/sec per megaparsec] will be different if you go back in the history of the universe. So, that is something we can measure and is very important for us to figure out what why the universe is expanding.

RK: In 2011, the Nobel Prize for Physics was awarded to three scientists for their discovery of the accelerating expansion of the Universe through observations of distant supernovae. They studied over 50 distant supernovae to suggest that the expansion of the Universe was accelerating. What is DESI doing differently and what advancements has the new survey brought in?

SA: Supernovae, or dramatic explosions that take place during the final stages of the death of a supermassive star, is a completely different way to measure expansion rates. We are also studying expansion using a different method.

Essentially, a couple of different methods can be ideally used to measure expansion rates. The supernova is like standard candle, so we know how much light they are giving away. Therefore, we can estimate distance to supernova, simply by looking at the amount of light we observe from them. And by looking at supernova at different distances, we can figure out how the universe is expanding. That’s the idea there. 

There are other methods like Cosmic Microwave Background (CMB), which is the cooled remnant of the first light that could have travelled freely throughout the Universe. It can also be used to measure Hubble constant. 

CMB measurement comes from the very early universe and supernova measurement comes from what we call the nearby universe.

DESI conducts galaxy survey, which acts like a standard ruler (there is a special scale called Baryon acoustic oscillation whose value is known and hence became a standard ruler). By looking at the standard ruler at different distances from Earth, we can again measure Hubble constant. We go back to as far as 11 billion years from us – the edge of our survey. To put this in context, the universe itself is about 13.6 billion years old.

Over the years, since the discovery of accelerated expansion, we have managed to perform detailed analysis which now gives us precise measurement. So, this is about the error bar. Our value is 68.5 (+/- 0.6). This is a very precise measurement of the Hubble constant.

Ideally, each of these methods should give us similar measurements. Over the last two decades, we have started to see these numbers disagree with each other.

If you look at supernova measurement, the Hubble constant is about 73 (+/- 1 roughly). For CMB, it is about 67 point something plus minus 0.8 or so. And then, what we have found is 68.5 plus minus 0.6. This is slightly above CMB, but it’s more consistent within the error bar. But it’s very discrepant with supernova measurements.

This is sort of is both worrisome and exciting. The discrepancies could mean two things. Maybe there is something missing in our analysis or it could mean that there is a new physics which is operating in the universe that has not been accounted for. So, there are lots of ideas in our community about what kind of physics we could unveil.

Our analysis is based on just a year of data and we’ll continue this for five years and at that time we’ll have five times more data. We expect to have much more precise measurement not just for the value of Hubble constant now but we’ll have very precise measurement of how the constant changes with time.

RK: Your results show subtle differences from the Standard Cosmological Model. What are these differences?

SA: The whole point of cosmological constant is basically saying that when the universe expands, the energy density of dark energy is the same. (Cosmological constant was introduced by Albert Einstein to suggest the existence of a repulsive force that counteracts the gravitational attraction holding matter together in space. In modern usage, the term ‘cosmological constant’ refers to a mathematical term added to the field equations of general relativity to give a consistent model of the universe). But the new data provides hints of change in energy density of dark energy.

If the energy density of dark energy changes, we cannot call it a cosmological constant. If there is any change in it at all, then you cannot call it a cosmological constant. So it cannot be described by same kind of ideas. 

You need to bring in new ideas. So, what we have done is measured the expansion rate as a function of time using what we call dark energy equation state. That’s basically how the dark energy density evolves. 

What we found is that it deviates from the value which you expect for cosmological constant, but it’s not at high significance. Again, the error bar: When you look at and account for the deviated value, it’s small and so we call this a hint. And we hope with future analysis of DESI, with three years of data and more, we can ascertain if this hint is a coincidence or some statistical fluctuation.

But now, we are seeing a sign that dark energy is not a cosmological constant. If we can prove this with more data, it will throw away many of the theoretical ideas that explain universe and bring in new ideas in terms of what is possibly the reason.