Nasa’s Webb, Hubble telescopes affirm universe’s expansion rate, puzzle persists

When you are trying to solve one of the biggest conundrums in cosmology, you should triple check your homework. The puzzle, called the "Hubble Tension," is that the current rate of the expansion of the universe is faster than what astronomers expect it to be, based on the universe's initial conditions and our present understanding of the universe's evolution.

Scientists using NASA's Hubble Space Telescope and many other telescopes consistently find a number that does not match predictions based on observations from ESA's (European Space Agency's) Planck mission. Does resolving this discrepancy require new physics? Or is it a result of measurement errors between the two different methods used to determine the rate of expansion of space?

Hubble has been measuring the current rate of the universe's expansion for 30 years, and astronomers want to eliminate any lingering doubt about its accuracy. Now, Hubble and NASA's James Webb Space Telescope have tag-teamed to produce definitive measurements, furthering the case that something else -- not measurement errors -- is influencing the expansion rate.

"With measurement errors negated, what remains is the real and exciting possibility we have misunderstood the universe," said Adam Riess, a physicist at Johns Hopkins University in Baltimore. Riess holds a Nobel Prize for co-discovering the fact that the universe's expansion is accelerating, due to a mysterious phenomenon now called "dark energy."

As a crosscheck, an initial Webb observation in 2023 confirmed that Hubble measurements of the expanding universe were accurate. However, hoping to relieve the Hubble Tension, some scientists speculated that unseen errors in the measurement may grow and become visible as we look deeper into the universe. In particular, stellar crowding could affect brightness measurements of more distant stars in a systematic way.

The SH0ES (Supernova H0 for the Equation of State of Dark Energy) team, led by Riess, obtained additional observations with Webb of objects that are critical cosmic milepost markers, known as Cepheid variable stars, which now can be correlated with the Hubble data.

"We've now spanned the whole range of what Hubble observed, and we can rule out a measurement error as the cause of the Hubble Tension with very high confidence," Riess said.

The team's first few Webb observations in 2023 were successful in showing Hubble was on the right track in firmly establishing the fidelity of the first rungs of the so-called cosmic distance ladder.

Astronomers use various methods to measure relative distances in the universe, depending upon the object being observed. Collectively these techniques are known as the cosmic distance ladder -- each rung or measurement technique relies upon the previous step for calibration.

But some astronomers suggested that, moving outward along the "second rung," the cosmic distance ladder might get shaky if the Cepheid measurements become less accurate with distance. Such inaccuracies could occur because the light of a Cepheid could blend with that of an adjacent star -- an effect that could become more pronounced with distance as stars crowd together and become harder to distinguish from one another.

The observational challenge is that past Hubble images of these more distant Cepheid variables look more huddled and overlapping with neighboring stars at ever farther distances between us and their host galaxies, requiring careful accounting for this effect. Intervening dust further complicates the certainty of the measurements in visible light. Webb slices though the dust and naturally isolates the Cepheids from neighboring stars because its vision is sharper than Hubble's at infrared wavelengths.

"Combining Webb and Hubble gives us the best of both worlds. We find that the Hubble measurements remain reliable as we climb farther along the cosmic distance ladder," said Riess.

The new Webb observations include five host galaxies of eight Type Ia supernovae containing a total of 1,000 Cepheids, and reach out to the farthest galaxy where Cepheids have been well measured -- NGC 5468 -- at a distance of 130 million light-years. "This spans the full range where we made measurements with Hubble. So, we've gone to the end of the second rung of the cosmic distance ladder," said co-author Gagandeep Anand of the Space Telescope Science Institute in Baltimore, which operates the Webb and Hubble telescopes for NASA.

Hubble and Webb's further confirmation of the Hubble Tension sets up other observatories to possibly settle the mystery. NASA's upcoming Nancy Grace Roman Space Telescope will do wide celestial surveys to study the influence of dark energy, the mysterious energy that is causing the expansion of the universe to accelerate. ESA's Euclid observatory, with NASA contributions, is pursuing a similar task.

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A new possible explanation for the Hubble tension

The universe is expanding. How fast it does so is described by the so-called Hubble-Lemaitre constant. But there is a dispute about how big this constant actually is: Different measurement methods provide contradictory values. This so-called "Hubble tension" poses a puzzle for cosmologists. Researchers from the Universities of Bonn and St. Andrews are now proposing a new solution: Using an alternative theory of gravity, the discrepancy in the measured values can be easily explained -- the Hubble tension disappears. The study has now been published in the Monthly Notices of the Royal Astronomical Society (MNRAS).

The expansion of the universe causes the galaxies to move away from each other.

The speed at which they do this is proportional to the distance between them.

For instance, if galaxy A is twice as far away from Earth as galaxy B, its distance from us also grows twice as fast.

The US astronomer Edwin Hubble was one of the first to recognize this connection.

In order to calculate how fast two galaxies are moving away from each other, it is therefore necessary to know how far apart they are.

However, this also requires a constant by which this distance must be multiplied.

This is the so-called Hubble-Lemaitre constant, a fundamental parameter in cosmology.

Its value can be determined, for example, by looking at the very distant regions of the universe.

This gives a speed of almost 244,000 kilometers per hour per megaparsec distance (one megaparsec is just over three million light years).

244.000 kilometers per hour per megaparsec -- or 264,000?

"But you can also look at celestial bodies that are much closer to us -- so-called category 1a supernovae, which are a certain type of exploding star," explains Prof.

Dr. Pavel Kroupa from the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn.

It is possible to determine the distance of a 1a supernova to Earth very precisely.

We also know that shining objects change color when they move away from us -- and the faster they move, the stronger the change.

This is similar to an ambulance, whose siren sounds deeper as it moves away from us.

If we now calculate the speed of the 1a supernovae from their color shift and correlate this with their distance, we arrive at a different value for the Hubble-Lemaitre constant -- namely just under 264,000 kilometers per hour per megaparsec distance.

"The universe therefore appears to be expanding faster in our vicinity -- that is, up to a distance of around three billion light years -- than in its entirety," says Kroupa.

"And that shouldn't really be the case."

However, there has recently been an observation that could explain this.

According to this, the Earth is located in a region of space where there is relatively little matter -- comparable to an air bubble in a cake.

The density of matter is higher around the bubble. Gravitational forces emanate from this surrounding matter, which pull the galaxies in the bubble towards the edges of the cavity.

"That's why they are moving away from us faster than would actually be expected," explains Dr. Indranil Banik from St. Andrews University.

The deviations could therefore simply be explained by a local "under-density."

In fact, another research group recently measured the average speed of a large number of galaxies that are 600 million light years away from us. "It was found that these galaxies are moving away from us four times faster than the standard model of cosmology allows," explains Sergij Mazurenko from Kroupa's research group, who was involved in the current study.

Bubble in the dough of the universe

This is because the standard model does not provide for such under-densities or "bubbles" -- they should not actually exist.

Instead, matter should be evenly distributed in space. If this were the case, however, it would be difficult to explain which forces propel the galaxies to their high speed.

"The standard model is based on a theory of the nature of gravity put forward by Albert Einstein," says Kroupa.

"However, the gravitational forces may behave differently than Einstein expected." The working groups from the Universities of Bonn and St. Andrews have used a modified theory of gravity in a computer simulation.

This "modified Newtonian dynamics" (abbreviation: MOND) was proposed four decades ago by the Israeli physicist Prof.

Dr. Mordehai Milgrom. It is still considered an outsider theory today.

"In our calculations, however, MOND does accurately predict the existence of such bubbles," says Kroupa.

If one were to assume that gravity actually behaves according to Milgrom's assumptions, the Hubble tension would disappear: There would actually only be one constant for the expansion of the universe, and the observed deviations would be due to irregularities in the distribution of matter.

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A new measurement could change our understanding of the Universe

The Universe is expanding -- but how fast exactly? The answer appears to depend on whether you estimate the cosmic expansion rate -- referred to as the Hubble's constant, or H0 -- based on the echo of the Big Bang (the cosmic microwave background, or CMB) or you measure H0 directly based on today's stars and galaxies. This problem, known as the Hubble tension, has puzzled astrophysicists and cosmologists around the world.

A study carried out by the Stellar Standard Candles and Distances research group, lead by Richard Anderson at EPFL's Institute of Physics, adds a new piece to the puzzle. Their research, published in Astronomy & Astrophysics, achieved the most accurate calibration of Cepheid stars -- a type of variable star whose luminosity fluctuates over a defined period -- for distance measurements to date based on data collected by the European Space Agency's (ESA's) Gaia mission. This new calibration further amplifies the Hubble tension.

The Hubble constant (H0) is named after the astrophysicist who, together with Georges Lemaître, discovered the phenomenon in the late 1920s. It's measured in kilometers per second per megaparsec (km/s/Mpc), where 1 Mpc is around 3.26 million light years.

The best direct measurement of H0 uses a "cosmic distance ladder," whose first rung is set by the absolute calibration of the brightness of Cepheids, now recalibrated by the EPFL study. In turn, Cepheids calibrate the next rung of the ladder, where supernovae -- powerful explosions of stars at the end of their lives -- trace the expansion of space itself. This distance ladder, measured by the Supernovae, H0, for the Equation of State of dark energy (SH0ES) team led by Adam Riess, winner of the 2011 Nobel Prize in Physics, puts H0 at 73.0 ± 1.0 km/s/Mpc.

First radiation after the Big Bang

H0 can also be determined by interpreting the CMB -- which is the ubiquitous microwave radiation left over from the Big Bang more than 13 billion years ago. However, this "early Universe" measurement method has to assume the most detailed physical understanding of how the Universe evolves, rendering it model dependent. The ESA's Planck satellite has provided the most complete data on the CMB, and according to this method, H0 is 67.4 ± 0.5 km/s/Mpc.

The Hubble tension refers to this discrepancy of 5.6 km/s/Mpc, depending on whether the CMB (early Universe) method or the distance ladder (late Universe) method is used. The implication, provided that the measurements performed in both methods are correct, is that there is something wrong in the understanding of the basic physical laws that govern the Universe. Naturally, this major issue underscores how essential it is for astrophysicists' methods to be reliable.

The new EPFL study is so important because it strengthens the first rung of the distance ladder by improving the calibration of Cepheids as distance tracers. Indeed, the new calibration allows us to measure astronomical distances to within ± 0.9%, and this lends strong support to the late Universe measurement. Additionally, the results obtained at EPFL, in collaboration with the SH0ES team, helped to refine the H0 measurement, resulting in improved precision and an increased significance of the Hubble tension.

"Our study confirms the 73 km/s/Mpc expansion rate, but more importantly, it also provides the most precise, reliable calibrations of Cepheids as tools to measure distances to date," says Anderson. "We developed a method that searched for Cepheids belonging to star clusters made up of several hundreds of stars by testing whether stars are moving together through the Milky Way. Thanks to this trick, we could take advantage of the best knowledge of Gaia's parallax measurements while benefiting from the gain in precision provided by the many cluster member stars. This has allowed us to push the accuracy of Gaia parallaxes to their limit and provides the firmest basis on which the distance ladder can be rested."

Rethinking basic concepts

Why does a difference of just a few km/s/Mpc matter, given the vast scale of the Universe? "This discrepancy has a huge significance," says Anderson. "Suppose you wanted to build a tunnel by digging into two opposite sides of a mountain. If you've understood the type of rock correctly and if your calculations are correct, then the two holes you're digging will meet in the center. But if they don't, that means you've made a mistake -- either your calculations are wrong or you're wrong about the type of rock. That's what's going on with the Hubble constant. The more confirmation we get that our calculations are accurate, the more we can conclude that the discrepancy means our understanding of the Universe is mistaken, that the Universe isn't quite as we thought."

The discrepancy has many other implications. It calls into question the very fundamentals, like the exact nature of dark energy, the time-space continuum, and gravity. "It means we have to rethink the basic concepts that form the foundation of our overall understanding of physics," says Anderson.

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