The cosmos is growing larger. The Hubble-Lemaitre constant describes how
quickly it does so. However, there is disagreement on the true value of this
constant: Conflicting results are obtained using various measuring
techniques.
For cosmologists, this "Hubble tension" is a riddle. A fresh approach is
now being put out by researchers from the Universities of Bonn and St.
Andrews: The Hubble tension vanishes when a different theory of gravity is
used, explaining the observed differences. The research has now been
released in the Royal Astronomical Society's Monthly Notices (MNRAS).
Galaxies separate from one another due to the universe's expansion. The
distance between them determines how quickly they do this. For example,
galaxy A's distance from Earth increases twice as quickly if it is twice as
far away as galaxy B. Edwin Hubble, an American astronomer, was among the
first to notice this link.
Consequently, knowing the distance between two galaxies is essential to
determining how quickly they are traveling apart. But in order to double
this distance, a constant is also needed. This is a key cosmological
parameter known as the Hubble-Lemaitre constant. One way to ascertain its
importance is to examine the furthest reaches of the cosmos. This results in
a speed of around 244,000 km/h for every megaparsec of distance (three
million light-years equals one megaparsec).
244.000 mph (megapascals per hour) or 264,000?
"However, you can also examine celestial objects that are considerably
closer to us—so-called category 1a supernovae, which are a specific kind of
burst star," says Prof. Dr. Pavel Kroupa of the University of Bonn's
Helmholtz Institute of Radiation and Nuclear Physics. The precise distance
of a 1a supernova from Earth may be ascertained. Additionally, we know that
as sparkling things travel away from us, their color changes, and the
stronger the change, the quicker they go. This sounds like an ambulance,
which gets farther away from us and makes a deeper siren.
We get a different value for the Hubble-Lemaitre constant if we now compute
the speed of the 1a supernovae based on their color shift and connect this
with their distance: little less than 264,000 km/s. "The universe,
therefore, appears to be expanding faster in our vicinity—that is, up to a
distance of around three billion light years—than in its entirety," Kroupa
explains. "And that shouldn't really be the case."
But a new finding could provide an explanation for this. This suggests that
the Earth is situated in an area of space where matter is scarce, like an
air bubble in a cake. The area surrounding the bubble has a greater matter
density. This surrounding stuff emits gravitational forces that push the
galaxies within the bubble toward the cavity's boundaries. According to Dr.
Indranil Banik of St. Andrews University, "that's why they are moving away
from us faster than would actually be expected." Thus, a simple explanation
for the variations would be a local "under-density."
Indeed, the average speed of a significant number of galaxies that are 600
million light years away from us was recently observed by another study
team. As Sergij Mazurenko from Kroupa's research group puts it, "it was
found that these galaxies are moving away from us four times faster than the
standard model of cosmology allows."
A bubble in the universe's dough
This is due to the fact that such under-densities or "bubbles" are not
allowed for in the usual model and shouldn't genuinely exist. Rather, matter
ought to be dispersed uniformly throughout space. But if that were the true,
it would be challenging to explain what factors cause the galaxies to
accelerate to such a great pace.
"The standard model is based on a theory of the nature of gravity put
forward by Albert Einstein," Kroupa explains. "However, the gravitational
forces may behave differently than Einstein expected." In a computer
simulation, the working groups from the Universities of Bonn and St. Andrews
have used a modified theory of gravity.
The term "modified Newtonian dynamics" (abbreviated: MOND) was first used
forty years ago by Prof. Dr. Mordehai Milgrom, an Israeli psychologist. Even
now, it is still regarded as a fringe hypothesis. "In our calculations,
however, MOND does accurately predict the existence of such bubbles," Kroupa
states.
The Hubble tension would vanish if gravity truly operated in accordance
with Milgrom's theories: the universe's expansion would be governed by a
single constant, and the observable variations would result from anomalies
in the distribution of matter.
