An Expert Explains: How James Webb Space Telescope has raised questions in cosmology

With its huge mirror — five times bigger than that of its predecessor, the Hubble Space Telescope — the JWST, which was launched on Christmas Day in 2021 and arrived at its destination, the Sun-Earth Lagrange point 2 in January 2022, has been peering at the early epochs in the history of the Universe, when the first galaxies had barely formed.

Its images were, however, very different from what astronomers had thought they would see.

They had expected to find newborn galaxies — instead, the early phases of the Universe appear to be full of adult-sized galaxies. And if that is not enough of a puzzle, the rate at which the Universe is expanding, as measured by the space telescope, appears to be at odds with other pieces of data.

Ways to measure expansion

The rate of expansion of the Universe has been a subject of scientific debate for quite some time. Two different methods of determining the rate have yielded results that differ by as much as 10%.

One method is based on phenomena in the early Universe — which implies events at a great distance, because the light we see from distant objects started its journey a long time ago. The other method relies on local celestial objects — although ‘local’ means a region spanning billions of light years.

Some stars vary their brightness in a periodic manner, and the duration of this change tells us something about how bright they really are. From this, one can figure out their distance and, in turn, how the Universe has been expanding.

The new space telescope was expected to nail the reason for the mismatch between the results obtained by the two methods. But its measurements seem to have only increased the discrepancy. Its deeper inspection of the local method of measurement gives a rate of expansion that is somewhat faster than that based on early Universe measurements.

The question is, which method is more reliable?

The early Universe method relies on a relic radiation from the primeval epochs, when the Universe was hot. The radiation has now cooled down as the Universe has expanded, and has become a microwave ‘hum’ that pervades everything. If you make a fist, hundreds of photons from this relic radiation will pass through it every second!

A detailed analysis of this radiation can tell us how fast the Universe has been expanding. In other words, one method depends on how well we know our stars, and the other on how well we understand the history of the Universe.

‘Adults in a kindergarten’

There have been problems with the early history of the Universe as well.

The space telescope was designed to peer at toddler galaxies. Galaxies such as our Milky Way contain roughly a hundred billion stars. But around 13 billion years ago, when the Universe was less than a tenth of its current age, small clumps of matter began to gather together and create suitable conditions for lighting up the first stars.

Those baby galaxies were supposed to be relatively small. But the data coming out of JWST seem to show full-bodied galaxies at the dawn of the Universe! Most of them seem to contain billions of stars. It has been as mind-boggling as, say, finding a bunch of adults playing in a kindergarten.

Some galaxies that JWST has imaged and analysed date back to when the Universe was barely a few hundred million years old. Yet, they seem to contain billions of majestically bright stars. They could have been exceptions, but then such objects should be rare — which they are not. Even if they are to be treated as exceptions, one would have to come up with an explanation for such exuberant formation of stars at the dawn of the Universe. Although astronomers still hope to find some explanation, even if one has to push the limits of astrophysical processes, some have begun to wonder if there was something wrong after all with our basic ideas of the Universe.

Crossroads of knowledge

There is only one Universe. And we are a part of it. We can’t change it, tweak it, or experiment with it. Which makes it unlike any other topic of research in science. Yet, beginning with the last century, equipped with Einstein’s theory of gravitation and large telescopes, physicists made ambitious attempts to study and explain the Universe.

Over the decades, a ‘standard model’ of the Universe has emerged, which is appealing in its simplicity, and also flexible enough to accommodate complexity. Based on it, a history of the Universe has been sketched out — barring the very first moment, which remains elusive.

However, the so-called ‘standard model’ has needed quick fixes in more than one aspect. Faced with a mismatch between the inventory of matter and the observed pull of gravity, scientists have invented “dark matter”, although no one has any idea what it is made of. The Universe also appears to be in a hurry while expanding. In order to explain this, we have brought in “dark energy” — but we remain clueless about its nature.

Some astronomers have begun to think that the gaps in our understanding may not be just a matter of details — and may warrant a comprehensive revision of what we think of the Universe. There have been occasions in the history of science when scientists have held on to existing ideas by introducing patchwork to accommodate contradictions between theory and observations. The Greeks for example, kept introducing complexities to the orbits of planets around the Earth, disregarding suggestions that their basic geocentric model of the Universe was wrong.

Scientists sometimes behave as though their conventional model is the only possibility, and ignore the conflicting measurements, until a tectonic shift changes everything. It is not possible to predict how, if at all, a paradigm shift will take place. Only time will tell if the puzzling results from the space telescope will be explained away by another patchwork, by inventing another ‘dark’ something, or if they will lead to a rethink of cosmic history.

The author is an astrophysicist at the Raman Research Institute, Bangalore.