A new study, published on April 25 in the Astrophysical Journal, has revealed that our universe is expanding alarmingly faster than expected, effectively raising more questions about one of the biggest mysteries in astronomy than answering them
Although astronomers have known all along that the universe has been expanding ever since the big bang more than 13 billion years ago, the fact that it is growing about nine percent faster than earlier predictions, as the new Hubble measurements suggest, calls for new theories to better understand the forces that have shaped the cosmos.
The difference in the expansion rate of the modern universe and the measurements of the early universe (based on estimates from the European Space Agency’s Planck satellite), has been the subject of many a scientific paper over the last several years.
However, the disparity reflected this time around is far too significant to pass it off as a fluke or blame it on different measurement techniques.
Hubble constant, or the rate at which the universe is expanding, is prone to discrepancies, depending on the method used by scientists to measure it, but the latest findings have reduced the probability of the disparity being a fluke from 1 in 3,000 to 1 in 100,000.
“The Hubble tension between the early and late universe may be the most exciting development in cosmology in decades,” said Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, in Baltimore, Maryland.
This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This disparity could not plausibly occur just by chance,” Reiss, who is also the lead researcher of the study, added.
The Planck technique measures the expansion at around 67 kilometers (41.6 miles) per second per megaparsec, which means for every 3.26 million light-years farther away a galaxy is from us, the expansion of the universe is causing it to move 67 kilometers per second faster.
The technique involves mapping of the cosmic microwave background (CMB), or the condition of the universe as it was 380, 000 years after the big bang – “a relic afterglow” as NASA describes it.
The Hubble method calculated the Hubble constant at 74 kilometers (46 miles) per second per megaparsec – the difference between the two measurements being the nine percent disparity in question.
The Hubble Space Telescope method of calculating the Hubble constant involves three basic steps, all of which require building a “cosmic distance ladder.”
To start with, accurate distances to neighboring galaxies are measured, moving farther and farther away to distant galaxies, building the so-called “cosmic distance ladder” in the process.
“This “ladder” is a series of measurements of different kinds of astronomical objects with an intrinsic brightness that researchers can use to calculate distances,” explains NASA.
“Among the most reliable for shorter distances are Cepheid variables, stars that pulsate at predictable rates that indicate their intrinsic brightness,” says the space agency.
New observations of 70 Cepheid variables in the Large Magellanic Cloud, a nearby satellite galaxy, allowed the astronomers to compare the measurements of these Cepheid variables to those in more distant galaxies, including exploding stars called Type Ia supernovas.
Since supernovas are much brighter than Cepheids, astronomers use them as “milepost markers” to calculate the distance to galaxies that are farther away in the outer reaches of the universe.
Each marker represents a rung in the “cosmic distance ladder,” which can be extended by adding more reliable markers, thereby enabling astronomers to reach farther and farther away to far-flung galaxies.
The distances to these markers are then compared to measurements of the reddish glow emanating from an entire galaxy, the redness increasing with distance – a result of the uniform expansion of the universe.
Astronomers can then work out the rate at which the universe is expanding.
“When Hubble uses precise pointing by locking onto guide stars, it can only observe one Cepheid per each 90-minute Hubble orbit around Earth. So, it would be very costly for the telescope to observe each Cepheid,” said Stefano Casertano, one of the co-authors of the study – also from STScI and Johns Hopkins.
“Instead, we searched for groups of Cepheids close enough to each other that we could move between them without recalibrating the telescope pointing,” he explained.
“These Cepheids are so bright, we only need to observe them for two seconds,” Casertano said, adding that the technique was allowing the team “to observe a dozen Cepheids for the duration of one orbit.”
As to why the universe is expanding at such a rapid pace is still a burning question for the astronomers which requires further research.
There are, however, a few “dark” theories, including the “early dark energy”, the “dark radiation” and the “dark matter” theories, that attempt to explain the disparity.
NASA’s April 25 article states:
“Astronomers have already hypothesized that dark energy existed during the first seconds after the big bang and pushed matter throughout space, starting the initial expansion.
“Dark energy may also be the reason for the universe’s accelerated expansion today. The new theory suggests that there was a third dark-energy episode not long after the big bang, which expanded the universe faster than astronomers had predicted.
“The existence of this “early dark energy” could account for the tension between the two Hubble constant values, Riess said.”
The other explanation for the mismatch is the presence of a new subatomic particle in space which travels close to the speed of light; collectively, these fast-moving particles are known as “dark radiation.”
Previously known particles, including neutrinos, (created in nuclear reactions and radioactive decays) are also part of this dark radiation.
As for “dark matter,” although it exists only in theory, scientists strongly believe that it is an all-pervasive reality in galaxy clusters, accounting for 85 percent of all matter in the known and unknown universe.
Their conviction is based on astrophysical observations such as unexplained gravitational forces, which, obviously, can’t come from anything.
Meaning, while they can see the powerful gravitational effects of the so-called dark matter, they can’t really see the matter itself; hence, the name.
A recent study, however, claims to have found a way to track the dark matter.
Using deep-space imagery captured by the Hubble Telescope, astronomers Mireia Montes (School of Physics, University of New South Wales, Australia) and Ignacio Trujillo (Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain) were able to see the invisible matter in an unprecedented light, literally.