Key Takeaways:

Albert Einstein’s theory of general relativity has been remarkably successful in describing the gravity of stars and planets. However, gaps in our understanding start to appear when we try to apply it to extremely small distances, where the laws of quantum mechanics operate. A new study, published in Nature Astronomy, has now tested Einstein’s theory on the largest of scales. We believe our approach may one day help resolve some of the biggest mysteries in cosmology, and the results hint that the theory may need to be tweaked on this scale.


Everything in the universe has gravity – and feels it too. Yet this most common of all fundamental forces is also the one that presents the biggest challenges to physicists. Albert Einstein’s theory of general relativity has been remarkably successful in describing the gravity of stars and planets, but it doesn’t seem to apply perfectly on all scales.

General relativity has passed many years of observational tests, from Eddington’s measurement of the deflection of starlight by the Sun in 1919 to the recent detection of gravitational waves. However, gaps in our understanding start to appear when we try to apply it to extremely small distances, where the laws of quantum mechanics operate, or when we try to describe the entire universe.

Our new study, published in Nature Astronomy, has now tested Einstein’s theory on the largest of scales. We believe our approach may one day help resolve some of the biggest mysteries in cosmology, and the results hint that the theory of general relativity may need to be tweaked on this scale.

Faulty model?

Quantum theory predicts that empty space, the vacuum, is packed with energy. We do not notice its presence because our devices can only measure changes in energy rather than its total amount. 

However, according to Einstein, the vacuum energy has a repulsive gravity – it pushes the empty space apart. Interestingly, in 1998, it was discovered that the expansion of the universe is in fact accelerating (a finding awarded with the  2011 Nobel prize in physics). However, the amount of vacuum energy, or dark energy as it has been called, necessary to explain the acceleration is many orders of magnitude smaller than what quantum theory predicts. 

Hence the big question, dubbed “the old cosmological constant problem”, is whether the vacuum energy actually gravitates – exerting a gravitational force and changing the expansion of the universe.

If yes, then why is its gravity so much weaker than predicted? If the vacuum does not gravitate at all, what is causing the cosmic acceleration? 

We don’t know what dark energy is, but we need to assume it exists in order to explain the universe’s expansion. Similarly, we also need to assume there is a type of invisible matter presence, dubbed dark matter, to explain how galaxies and clusters evolved to be the way we observe them today.

These assumptions are baked into scientists’ standard cosmological theory, called the lambda cold dark matter (LCDM) model – suggesting there is 70% dark energy, 25% dark matter and 5% ordinary matter in the cosmos. And this model has been remarkably successful in fitting all the data collected by cosmologists over the past 20 years.

But the fact that most of the universe is made up of dark forces and substances, taking odd values that don’t make sense, has prompted many physicists to wonder if Einstein’s theory of gravity needs modification to describe the entire universe.

A new twist appeared a few years ago when it became apparent that different ways of measuring the rate of cosmic expansion, dubbed the Hubble constant, give different answers – a problem known as the Hubble tension.

The disagreement, or tension, is between two values of the Hubble constant. One is the number predicted by the LCDM cosmological model, which has been developed to match the light left over from the Big Bang (the cosmic microwave background radiation). The other is the expansion rate measured by observing exploding stars known as supernovas in distant galaxies. 

Image of the cosmic microwave background.
Cosmic microwave background. Nasa

Many theoretical ideas have been proposed for ways of modifying LCDM to explain the Hubble tension. Among them are alternative gravity theories.

Digging for answers

We can design tests to check if the universe obeys the rules of Einstein’s theory. General relativity describes gravity as the curving or warping of space and time, bending the pathways along which light and matter travel. Importantly, it predicts that the trajectories of light rays and matter should be bent by gravity in the same way.

Together with a team of cosmologists, we put the basic laws of general relativity to test. We also explored whether modifying Einstein’s theory could help resolve some of the open problems of cosmology, such as the Hubble tension. 

To find out whether general relativity is correct on large scales, we set out, for the first time, to simultaneously investigate three aspects of it. These were the expansion of the universe, the effects of gravity on light and the effects of gravity on matter. 

Using a statistical method known as the Bayesian inference, we reconstructed the gravity of the universe through cosmic history in a computer model based on these three parameters. We could estimate the parameters using the cosmic microwave background data from the Planck satellite, supernova catalogues as well as observations of the shapes and distribution of distant galaxies by the SDSS and DES telescopes. We then compared our reconstruction to the prediction of the LCDM model (essentially Einstein’s model). 

We found interesting hints of a possible mismatch with Einstein’s prediction, albeit with rather low statistical significance. This means that there is nevertheless a possibility that gravity works differently on large scales, and that the theory of general relativity may need to be tweaked. 

Our study also found that it is very difficult to solve the Hubble tension problem by only changing the theory of gravity. The full solution would probably require a new ingredient in the cosmological model, present before the time when protons and electrons first combined to form hydrogen just after the Big Bang, such as a special form of dark matter, an early type of dark energy or primordial magnetic fields. Or, perhaps, there’s a yet unknown systematic error in the data.

That said, our study has demonstrated that it is possible to test the validity of general relativity over cosmological distances using observational data. While we haven’t yet solved the Hubble problem, we will have a lot more data from new probes in a few years.

This means that we will be able to use these statistical methods to continue tweaking general relativity, exploring the limits of modifications, to pave the way to resolving some of the open challenges in cosmology.

Contributor

Recently Published

Key Takeaway: Airports often lead to unusual behavior, such as drunken brawls and flight diversions. The psychology of travel can influence this behavior, as the promise of vacation can shift our mindset into anticipation and liberation. The overwhelming noise, crowds, and constant hustle can overload our senses, leading to stress-induced irritability and unexpected outbursts. Airports […]
Key Takeaway: Over 8,500 shipwrecks from two devastating world wars pose a significant threat to marine ecosystems and coastal communities. These underwater time capsules contain up to 6 billion gallons of oil, munitions, toxic heavy metals, and chemical weapons. The degradation of these wrecks is accelerating due to rising ocean temperatures, increasing acidification, and more […]

Top Picks

Key Takeaway: South Korea’s cultural exports have surged to unprecedented heights, with the latest season of “Squid Game” solidifying its position as a global entertainment powerhouse. The “Korean Wave” or Hallyu, which includes TV series, movies, animation, and music, has generated approximately US$3.7 billion annually for South Korea through exports, consumer spending, and tourism. The […]
Key Takeaway: A study published in Nature Astronomy found a strong consensus among scientists that extraterrestrial life exists in the universe. The survey, which involved 521 astrobiologists and 534 non-astrobiologists, found that 86.6% agreed or strongly agreed that some form of extraterrestrial life exists. The study also found that 67.4% of astrobiologists and 58.2% of […]
Key Takeaway: Researchers are using genome-scale metabolic models (GEMs) to study the complexities of microbial life, offering innovative solutions to climate change and space challenges. GEMs simulate the vast network of metabolic pathways within living organisms, allowing scientists to test and predict microbial behavior across diverse environments. They provide a detailed framework for understanding organisms’ […]
Key Takeaway: The “Wirkin” bag, a $78 imitation of Hermès’ Birkin bag, has gained popularity on TikTok, attracting millions of users. The bag, a clever portmanteau of “Walmart” and “Birkin,” allows everyday consumers to partake in high-end fashion without the exorbitant cost. The trend reflects a broader shift in societal values, where symbols of wealth […]
Key Takeaway: In 2022, scientists achieved the “experiment of the century” at Lawrence Livermore National Laboratory, marking the first time a fusion reaction produced more energy than consumed. This achievement has attracted billions of dollars in private investment, particularly in the United States, to advance fusion technology. However, a myriad of engineering challenges remain before […]

Trending

I highly recommend reading the McKinsey Global Institute’s new report, “Reskilling China: Transforming The World’s Largest Workforce Into Lifelong Learners”, which focuses on the country’s biggest employment challenge, re-training its workforce and the adoption of practices such as lifelong learning to address the growing digital transformation of its productive fabric. How to transform the country […]

Join our Newsletter

Get our monthly recap with the latest news, articles and resources.

Login

Welcome to Empirics

We are glad you have decided to join our mission of gathering the collective knowledge of Asia!
Join Empirics