- Our universe seems to be expanding faster and faster with time
- ‘Dark energy’ proposed as an explanation, but its nature remains a mystery
- Hubble observations have ruled out one dark energy hypothesis
ASTRONOMERS USING NASA’s Hubble Space Telescope have ruled out one explanation for the nature of dark energy after recalculating the expansion rate of the universe to unprecedented accuracy.
The universe appears to be expanding at an increasing rate. Some think this is because it is filled with a ‘dark energy’ that works in the opposite way to gravity.
An alternative to that hypothesis is that an enormous ‘bubble’ of relatively empty space eight billion light-years wide surrounds our galactic neighbourhood.
If we lived near the centre of this void, observations of galaxies being pushed away from each other at accelerating speeds would be an illusion.
This hypothesis has now been invalidated because astronomers have refined their understanding of the universe’s present expansion rate.
“We are using the new camera on Hubble like a policeman’s radar gun to catch the universe speeding,” said Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, and leader of the science team. “It looks more like it’s dark energy that’s pressing the [accelerator] pedal.”
The observations helped determine a figure for the universe’s current expansion rate to an uncertainty of just 3.3 percent. The new measurement reduces the error margin by 30 percent over Hubble’s previous best measurement in 2009.
Riess’ team first had to determine accurate distances to galaxies near and far from Earth, and then compare those distances with the speed at which the galaxies are apparently receding because of the expansion of space.
They used those two values to calculate the Hubble constant, the number that relates the speed at which a galaxy appears to recede to its distance from the Milky Way.
Because we cannot physically measure the distances to galaxies, astronomers have to find stars or other objects that serve as reliable cosmic yardsticks. These are objects with known intrinsic brightness—brightness that hasn’t been dimmed by distance, an atmosphere or interstellar dust. Their distances, therefore, can be inferred by comparing their intrinsic brightness with their apparent brightness as seen from Earth.
To calculate long distances, Riess’ team chose a special class of exploding star called Type Ia supernovae. These stellar blasts all have similar luminosity and are brilliant enough to be seen far across the universe.
By cross-correlating the apparent brightness of Type Ia supernovae with pulsating Cepheid stars (another class of stars whose intrinsic brightness is known), the team could accurately gauge the distances to Type Ia supernovae in far-flung galaxies.
Bubble is burst
By using the sharpness of Hubble’s new Wide Field Camera 3 (WFC3) to study more stars in visible and near-infrared light, the team eliminated systematic errors introduced by comparing measurements from different telescopes.
“WFC3 is the best camera ever flown on Hubble for making these measurements, improving the precision of prior measurements in a small fraction of the time it previously took,” said Lucas Macri, a collaborator on the Supernova Ho for the Equation of State (SHOES) Team from Texas A&M in College Station.
Knowing the precise value of the universe’s expansion rate further restricts the range of dark energy’s strength and helps astronomers tighten up their estimates of other cosmic properties, including the universe’s shape and its roster of neutrinos, or ghostly particles, that filled the early cosmos.
“Thomas Edison once said ‘every wrong attempt discarded is a step forward,’ and this principle still governs how scientists approach the mysteries of the cosmos,” said Jon Morse, astrophysics division director at NASA Headquarters in Washington.
“By falsifying the bubble hypothesis of the accelerating expansion, NASA missions like Hubble bring us closer to the ultimate goal of understanding this remarkable property of our universe.”
Adapted from information issued by STScI. NGC 5584 image credit: NASA / ESA / A. Riess (STScI/JHU), L. Macri (Texas A&M University) / Hubble Heritage Team (STScI/AURA). NGC 5584 illustrations credit: NASA / ESA / L. Frattare (STScI) / Z. Levay (STScI).
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