International Collaboration Helps Pinpoint Universe’s Expansion Rate - The Latest in NASA Science News

The universe is expanding, but the exact speed at which it’s doing so – a figure known as the “Hubble constant” – remains a mystery. Astronomers’ calculations have become far more precise over time, thanks to data from NASA’s Hubble and, more recently, James Webb space telescopes. But different approaches continue to offer slightly different results. Now, an international collaboration of astronomers has developed a framework that incorporates a range of methods into one analysis, to produce a more precise measurement.

To measure the Hubble constant, astronomers need to know the distance to various objects, and they use different techniques to measure that distance depending on how far away the objects are. The connection and calibration of one technique to the next is referred to as the “cosmic distance ladder.” Now, the new analysis incorporates a variety of these different distance markers, taking into account their relative robustness to minimize the impact of uncertainties on the distance ladder and deliver a more exact figure for the Hubble constant. The findings, published April 10 in the journal Astronomy & Astrophysics, indicate an expansion rate for the nearby universe of about 73.5 kilometers per second per megaparsec. Other estimates have typically ranged from 73 to 76 kilometers per second per megaparsec.

“The power of this work is that it doesn’t depend on any single method,” said Adam Riess, coauthor of the paper at the Johns Hopkins University and Space Telescope Science Institute in Baltimore. “When multiple, independent measurements all point to the same answer, it strengthens the case that we’re seeing a real feature of the universe, not a flaw in one technique. Right now, those measurements suggest the universe today is expanding faster than we would expect based on how it looked shortly after the big bang.”

One of NASA’s key goals in building Hubble decades ago was to determine the Hubble constant, which was theorized at the time to be different than observations of the early universe indicated. That disagreement is known as the Hubble tension. NASA will take the next step as early as this fall with the launch of the agency’s Nancy Grace Roman Space Telescope, which will deliver a vast wealth of data to improve distance indicators and further investigate the Hubble tension – a missing puzzle piece in our understanding of the cosmos.

This graphic provides an overview of the Local Distance Network, the research team’s new tool to integrate diverse measurements of the Hubble constant into a coherent, rig-orous framework.

Credit: Fabio Crameri (ISSI Bern), based on the original by Richard I. Anderson and the H₀DN Collaboration (2025). 

Source: International Collaboration Helps Pinpoint Universe’s Expansion Rate - NASA Science

Satellites reveal city methane emissions are rising faster than official estimates - Earth - Earth Sciences - Environment

The TROPOMI instrument aboard the European Sentinel-5P satellite orbits the earth and measures methane, a potent greenhouse gas. The bubbles show example satellite images of methane emissions over urban areas. Higher methane concentrations are depicted in pixels with warmer colors. Credit: Erica Whiting / University of Michigan Engineering

Urban emissions of methane—a potent greenhouse gas—are rising faster than bottom-up accounting estimates anticipated, according to a study led by University of Michigan Engineering. The discrepancy was found with satellite measurements of methane over 92 major cities around the world. For 72 of the cities, there were sufficient data to track changes in methane emissions between 2019 and 2023. Overall, global urban methane emissions in 2023 were 6% higher than 2019 levels and 10% higher than 2020 levels, although they tended to decrease in European cities.

In contrast, accounting methods—which tally emission estimates of individual methane sources—suggest that urban methane emissions have only risen between 1.7% and 3.7% since 2020. The work is published in the journal Proceedings of the National Academy of Sciences.

The study included over half of the C40 network, a group of 97 cities around the world aiming to reach net-zero emissions by 2050. Total methane emissions across all the studied C40 cities in 2023 were also 10% higher than 2020 levels, and the cities will have to contend with an extra two teragrams of methane emissions per year, which is about 30% of their emission reduction target. The gap between official estimates and satellite measurements warns that city policies designed with accounting estimates may not reduce methane emissions as desired.

"In order to reduce greenhouse gas emissions and set a good emissions policy, cities need to know how much they are emitting and what those sources are. But there is quite a bit of uncertainty with that for methane," said Eric Kort, corresponding author of the study. He advised the study's lead author as a U-M professor of climate and space sciences and engineering, and is now director of the Atmospheric Chemistry Department at the Max Planck Institute for Chemistry.

Checking emission books

The study continues Kort's work identifying gaps in accounting of methane, which can enter the atmosphere from old or leaky natural gas infrastructure, landfills and wastewater treatment plants, and is 80 times more potent at warming the planet than carbon dioxide over a 20-year period.

With measurements from airplane surveys, Kort's research group has shown that flaring at oil and gas production sites leaks 5 times more methane than previously estimated, and that the true climate impact of offshore oil and gas production is double the official estimates. The findings helped make flares an emissions-reduction target in the Inflation Reduction Act, leading to a $30 million Department of Energy call for new technology to reduce leaks from gas flaring.

In 2019, similar aerial measurements suggested that several large cities across the U.S. were also emitting more methane than previously thought. The new study showed that this is a global problem.

"Cities have the motivation and power to reduce greenhouse gas emissions and therefore, present significant opportunities for impactful emissions reduction," said Erica Whiting, U-M doctoral student in climate and space sciences and engineering and the study's first author. "However, there was not previously a method to quantify and monitor urban methane emissions around the globe, and therefore, no observation-based method to evaluate emission reduction strategies."

The researchers' global satellite measurements suggest that urban emissions accounted for 10% of all human methane emissions in 2023, and city methane emissions overall were nearly four times higher than the oil and gas "ultra emitters" that have been the focus of previous studies and emission policies.

Monitoring methane from space

The new findings come from the TROPOMI instrument, which was launched aboard the European Copernicus Sentinel-5 Precursor satellite in 2017 to track atmospheric pollution and climate change. TROPOMI measures the amount of sunlight reflected by the atmosphere back into space. It separately measures many wavelengths of light, each of which provides information on the concentration of a particular gas or pollutant, and it has sufficient spatial resolution to pinpoint individual cities.

TROPOMI's resolution is too coarse to identify where exactly unreported methane is coming from within the city, however. The researchers think that higher-resolution measurements could help cities update their accounts and emission policies.

"We, and others in the field, are looking into higher-resolution satellite measurements so that we can tease apart the contribution of large localized sources," Kort said. "Those satellites can't necessarily tell you the whole city's emissions, but they could tell you what individual landfills or facilities are doing." 

Provided by University of Michigan 

by Derek Smith, University of Michigan

edited by Stephanie Baum, reviewed by Robert Egan 

Source: Satellites reveal city methane emissions are rising faster than official estimates