For about a century, scientists have known that the Universe is in a state of constant expansion. In honor of the scientists who definitively showed this, this expansion has come to be known as the Hubble Constant (or Hubble-Lemaitre Constant). Today, scientists use two main techniques to measure the rate of expansion: the Cosmic Microwave Background (CMB) and the Cosmic Distance Ladder. The former relies on redshift measurements of the CMB, the relic radiation left over from the Big Bang, while the latter relies on parallax and redshift measurements using variable stars and supernovae (aka “standard candles”).

The only problem is that the two methods don’t agree, leading to what is known as the “Hubble Tension.” This problem is considered one of the greatest cosmological mysteries facing scientists today. Luckily, new methods are emerging that could help resolve this “tension” and bring order to the Standard Model of Cosmology. In a recent study, a team of astrophysicists, cosmologists, and physicists from the University of Illinois and the University of Chicago has proposed a new method using the tiny ripples in spacetime known as gravitational waves (GWs).

The study was led by Bryce Cousins, an NSF Graduate Research Fellow from the Institute of Gravitation and the Cosmos (IGC) at the University of Illinois Urbana-Champaign. He was joined by multiple colleagues from the IGC, as well as researchers from the Kavli Institute for Cosmological Physics and the Enrico Fermi Institute at the University of Chicago. Their study, “Stochastic Siren: Astrophysical gravitational-wave background measurements of the Hubble constant,” appeared on Jan. 16th in the Physical Review Letters.

Scientists hoping to resolve the Hubble Tension have proposed several solutions, ranging from Early Dark Energy (EDE) and interactions between Dark Matter (DM) and neutrinos to evolving dark-energy dynamics. In recent years, the discovery of gravitational waves has also emerged as a means of resolving the Tension by providing a new way to measure cosmic expansion. Originally predicted by Einstein’s Theory of General Relativity, gravitational waves are ripples caused in the fabric of spacetime caused by the merger of massive objects (neutron stars and/or black holes).

They were first confirmed in 2016 by scientists at the Laser Interferometer Gravitational wave Observatory (LIGO). Thanks to upgraded instruments and international cooperation, the LIGO-Virgo-KAGRA (LVK) collaboration has detected more than 300 GW events. In that time, astronomers have found ways to use events to explore cosmological phenomena, including measuring the expansion of the cosmos. In the current research, the team found a way to improve these measurements by leveraging the gravitational-wave background (GWB), which is caused by astrophysical collisions that the LVK network is not yet sensitive enough to detect.

They call it the “stochastic standard siren” method, since the collisions that make up the gravitational-wave background occur stochastically. Daniel Holz, a UChicago Professor and study co-author, explained in a UIUC press release:

It’s not every day that you come up with an entirely new tool for cosmology. We show that by using the background gravitational-wave hum from merging black holes in distant galaxies, we can learn about the age and composition of the universe. This is an exciting and completely new direction, and we look forward to applying our methods to future datasets to help constrain the Hubble constant, as well as other key cosmological quantities.

*Artist’s impression of the electromagnetic signal from the merger of two neutron stars. Credit: NSF/LIGO/Sonoma State University/A. Simonnet*

As a proof of principle, the team applied their method to current LVK Collaboration data. They found that the non-detection of the GWB provides evidence against slow cosmic expansion rates. They then combined their method with measurements of the Hubble Constant based on individual black hole collisions to obtain a more accurate rate. “Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the Universe,” said Cousins. “Based on those rates, we expect there to be a lot more events that we can’t observe, which is called the gravitational-wave background.”

This showed that at lower values of the Hubble constant, the total volume of space within which collisions occur is smaller. This would imply that the density of object collisions is higher, increasing the strength of the GWB signal to the point that current instruments could detect it. “This result is very significant—it’s important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension,” added co-author Nicolás Yunes, the founding director of the Illinois Center for Advanced Studies of the Universe (ICASU). “Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves.”

With LVK’s improved architecture, scientists estimate that the GWB is likely to be detected within the next six years. If and when this happens, the team’s method could be used to improve measurements of the Hubble Constant further. Until then, the stochastic siren method could be used to constrain higher values of the Hubble Constant, thereby establishing upper limits on the GWB and allowing scientists to study it before a full detection is made.

“This should pave the way for applying this method in the future as we continue to increase the sensitivity, better constrain the gravitational-wave background, and maybe even detect it,” says Cousins. “By including that information, we expect to get better cosmological results and be closer to resolving the Hubble tension.”

Further Reading: University of Illinois

A respiratory virus that doesn’t have a vaccine or a specific treatment regimen is spreading in some parts of California — but there’s no need to sound the alarm just yet, public health officials say.

A majority of Northern California communities have seen high concentrations of human metapneumovirus, or HMPV, detected in their wastewater, according to data from the WastewaterScan Dashboard, a public database that monitors sewage to track the presence of infectious diseases.

A Los Angeles Times data analysis found the communities of Merced in the San Joaquin Valley, and Novato and Sunnyvale in the San Francisco Bay Area have seen increases in HMPV levels in their wastewater between mid-December and the end of February.

HMPV has also been detected in L.A. County, though at levels considered low to moderate at this point, data show.

While HMPV may not necessarily ring a bell, it isn’t a new virus. Its typical pattern of seasonal spread was upended by the COVID-19 pandemic, and its resurgence could signal a return to a more typical pre-coronavirus respiratory disease landscape.

Here’s what you need to know.

What is HMPV?

HMPV was first detected in 2001, according to the U.S. Centers for Disease Control and Prevention. It’s transmitted by close contact with someone who is infected or by touching a contaminated surface, said Dr. Neha Nanda, chief of infectious diseases and hospital epidemiologist for Keck Medicine of USC.

Like other respiratory illnesses, such as influenza, HMPV spreads and is more durable in colder temperatures, infectious-disease experts say.

Human metapneumovirus cases commonly start showing up in January before peaking in March or April and then tailing off in June, said Dr. Jessica August, chief of infectious diseases at Kaiser Permanente Santa Rosa.

However, as was the case with many respiratory viruses, COVID disrupted that seasonal trend.

Why are we talking about HMPV now?

Before the pandemic hit in 2020, Americans were regularly exposed to seasonal viruses like HMPV and developed a degree of natural immunity, August said.

That protection waned during the pandemic, as people stayed home or kept their distance from others. So when people resumed normal activities, they were more vulnerable to the virus. Unlike other viruses, there isn’t a vaccine for human metapneumovirus.

“That’s why after the pandemic we saw record-breaking childhood viral illnesses because we lacked the usual immunity that we had, just from lack of exposure,” August said. “All of that also led to longer viral seasons, more severe illness. But all of these things have settled down in many respects.”

In 2024, the national test positivity for HMPV peaked at 11.7% at the end of March, according to the National Respiratory and Enteric Virus Surveillance System. The following year’s peak was 7.15% in late April.

So far this year, the highest test positivity rate documented was 6.1%, reported on Feb. 21 — the most recent date for which complete data are available.

While the seasonal spread of viruses like HMPV is nothing new, people became more aware of infectious diseases and how to prevent them during the pandemic, and they’ve remained part of the public consciousness in the years since, August and Nanda said.

What are the symptoms of HMPV?

Most people won’t go to the doctor if they have HMPV because it typically causes mild, cold-like symptoms that include cough, fever, nasal congestion and sore throat.

HMPV infection can progress to:

• An asthma attack and reactive airway disease (wheezing and difficulty breathing)
• Middle ear infections behind the ear drum
• Croup, also known as “barking” cough — an infection of the vocal cords, windpipe and sometimes the larger airways in the lungs
• Bronchitis
• Fever

Anyone can contract human metapneumovirus, but those who are immunocompromised or have other underlying medical conditions are at particular risk of developing severe disease — including pneumonia. Young children and older adults are also considered higher-risk groups, Nanda said.

What is the treatment for HMPV?

There is no specified treatment protocol or antiviral medication for HMPV. However, it’s common for an infection to clear up on its own and treatment is mostly geared toward soothing symptoms, according to the American Lung Assn.

A doctor will likely send you home and tell you to rest and drink plenty of fluids, Nanda said.

If symptoms worsen, experts say you should contact your healthcare provider.

How to avoid contracting HMPV

Infectious-disease experts said the best way to avoid contracting HMPV is similar to preventing other respiratory illnesses.

The American Lung Assn.’s recommendations include:

• Wash your hands often with soap and water. If that’s not available, clean your hands with an alcohol-based hand sanitizer.
• Clean frequently touched surfaces.
• Crack open a window to improve air flow in crowded spaces.
• Avoid being around sick people if you can.
• Avoid touching your eyes, nose and mouth.

Assistant data and graphics editor Vanessa Martínez contributed to this report.