No dark energy?Cosmologists think this is impossible.A study has questioned the existence of a mysterious antigravity force called dark energy.
Then the cosmologists hit back.A study questioned the existence of dark energy in the universe, and then cosmologists hit backSN 2007af shines clearly near the lower right corner of the spiral galaxy NGC 5584.Dark energy, as mysterious as it sounds, has become an important part of cosmology.Evidence of this repulsive energy being pumped into space has been mounting since 1998.That year, astronomers discovered dark energy for the first time, and as time went on, it became an accelerator, accelerating the expansion of the universe.
As space expands, new space emerges, and at the same time, more of this repulsive energy causes space to expand faster.Two decades later, independent measurements agree that dark energy accounts for about 70 percent of the universe.It is so deeply embedded in our current understanding of the universe that we were surprised when a paper published in the journal astronomy and astrophysics questioned its existence.The four authors, including Oxford University physicist subir sarkar, performed their own analysis of data from hundreds of supernovae.The supernova explosion provided the first evidence of cosmic acceleration, a discovery that won three astronomers the 2011 Nobel Prize in physics.When Sarkar and his colleagues looked at supernovae, they did not see the universe accelerating uniformly in all directions under the influence of dark energy.Instead, they say the supernova looks like this, because our region of the universe is accelerating in a particular direction, roughly toward the constellation Sagittarius in the southern sky.A study questioned the existence of dark energy in the universe, and then cosmologists hit backAlmost immediately, outside experts began to quibble over the paper, finding obvious flaws in its methodology.
Now a paper in the astrophysical journal by two cosmologists has put forward these arguments.Author David Rubin and his university of Hawaii student Jessica Heitlauf detail four major problems with Sarkar's data processing."Is the universe accelerating?""Asked the title of their paper.All the signs still suggest that the answer is yes.Outside researchers praised the thorough dissection.Sarkar, however, disagreed with the criticism and posted a rebuttal.Cosmologists are unimpressed, arguing that supernovae alone provide important and reliable evidence of dark energy.Mobile shootingThe expansion of space extends the light and makes it red in color.The further away supernovae are from us, the redshift they appear to be, because their light must travel farther through the expanding space.If the universe were expanding at a constant rate, the redshift of a supernova would be proportional to its distance, and therefore to its brightness.But in an accelerated universe full of dark energy, space expanded more slowly in the past than it does now.This means that a supernova's light stretches less during its long journey to earth, because space expands so slowly most of the time.Supernovae at a distance show far less redshift than a universe without dark energy.In fact, the researchers found that the redshift and brightness of supernovae are measured in this way.
A study questioned the existence of dark energy in the universe, and then cosmologists hit backSarkar takes an unconventional approach to analysis.Normally, any study of supernova data must take into account the earth's motion: as the earth orbits the sun, the sun orbits the Milky Way, and the sun orbits the local galaxy, we and our telescopes hurtle through space at about 600 kilometers per second.Our absolute motion is toward a dense region near the constellation Sagittarius.So the light from that direction is affected by the doppler shift, making it look bluer than the light from the other side of the sky.
Correcting this motion and converting the supernova data into a stationary reference frame is standard practice, but Sarkar does not.If you don't subtract that motion, then it will put the same doppler shift into the supernova data, and most of the effect is due to the motion of the solar system.Nor did they take into account the fact that cosmic dust absorbs more blue light.Because of this, a supernova looks particularly blue in a relatively clean, dust-free region where there is less dust that would otherwise absorb its blue light.Less dust also means it will look brighter.As a result, distant supernovae that we observe with telescopes appear disproportionately blue and bright.If you don't control for the color dependence effect of dust, you can infer that nearby supernovae are redder and grayer, and distant supernovae are bluer and brighter, with less difference in brightness, and eventually you can infer that the acceleration of the expanding universe is decreasing.A study questioned the existence of dark energy in the universe, and then cosmologists hit backCombined with these and other unusual decisions, Sarkar's team was able to simulate the supernova data with the "dipole" term, which is the acceleration pointing in a single direction, and the "unipolar" term, which is small, perhaps even zero, to describe the uniform acceleration of dark energy.There are two other problems with this dipole model.First, the model contains a term for the speed at which the dipole acceleration falls to zero as you move away from the earth.
Sarkar narrowed that distance, meaning that their model was not tested with a large number of supernova samples.Secondly, the model does not satisfy the consistency test of the relation between the dipole subterms and the monopole subterms in the involved equations.Not exactly the sameBut Mr Sarkar thinks no modification of his analysis is needed.He and his team were quick to rebut their four-point argument, mainly by revisiting previous arguments.Repeated analyses of the data found that the data from the supernovae matched other evidence of cosmic acceleration.For years, dark energy has been inferred from ancient light known as the cosmic microwave background, fluctuations in cosmic density known as baryonic acoustic oscillations, gravitationally warped galaxy shapes, and the concentration of matter in the universe.
A study questioned the existence of dark energy in the universe, and then cosmologists hit backSarkar bases his work on the cosmological fitting problem.Calculations of cosmic parameters, such as the density of dark energy, tend to assume that the universe is smooth, averaging out its inhomogeneity, such as galaxies and voids.
The fitting question asks whether this approximation leads to false inferences about a constant value like a lambda, or whether it might even imply the existence of a lambda that does not exist.But new research on the subject, including a large simulation of the universe published this summer, negates that possibility.Heterogeneity can change lambda by 1% or 2%, but it can't eliminate it. It's impossible.