Synopsis: No privilege for earthly observers

The idea that the observed expansion of the universe is related to our unique vantage point—as opposed to dark energy—seems to be inconsistent with telescope data.
Synopsis figure
Credit: R. R. Caldwell, Phys. Rev. Lett. 100, 191302 (2008)

According to most cosmologists, there is nothing special about us as observers of the universe. Still, some theories shirk this so-called Copernican principle, suggesting that the universe is not homogenous. Rather, these theories explain the observed accelerated expansion of the universe without invoking dark energy, and instead assume we are near the center of a void, beyond which denser matter pulls outwards.

In a paper appearing in Physical Review Letters, Pengjie Zhang at the Shanghai Astronomical Observatory and Albert Stebbins at Fermilab show that a popular void model, and many others aiming to replace dark energy, don’t stand up against telescope observation.

Galaxy surveys show the universe is homogeneous, at least on length scales up to a gigaparsec. Zhang and Stebbins argue that if larger scale inhomogeneities exist, they should be detectable as a temperature shift in the cosmic microwave background—relic photons from about 400,000 years after the big bang—that occurs because of electron-photon (inverse Compton) scattering. Focusing on the “Hubble bubble” void model, they show that in such a scenario, some regions of the universe would expand faster than others, causing this temperature shift to be greater than what is expected. But telescopes that study the microwave background, such as the Atacama telescope in Chile or the South Pole telescope, don’t see such a large shift.

Though they can’t rule out more subtle violations of the Copernican principle, Zhang and Stebbins’ test sets a high bar for future models to pass. – Jessica Thomas


Features

More Features »

Announcements

More Announcements »

Subject Areas

AstrophysicsCosmology

Previous Synopsis

Strongly Correlated Materials

Majorana states thrive under interactions

Read More »

Next Synopsis

Related Articles

Synopsis: 2D Maps of Solar Wind
Astrophysics

Synopsis: 2D Maps of Solar Wind

Maps of solar wind velocities derived from satellite images of the Sun’s corona could help researchers improve solar wind models. Read More »

Synopsis: Ideal Mergers for Measuring Cosmic Expansion
Cosmology

Synopsis: Ideal Mergers for Measuring Cosmic Expansion

Among gravitational-wave sources, the merger of a neutron star and a black hole may provide the most precise way to measure how fast the Universe is expanding. Read More »

Synopsis: A Closer Look at Cosmic Dust
Cosmology

Synopsis: A Closer Look at Cosmic Dust

Simulations provide a detailed picture of the emission of dust grains in our Galaxy, which is known to interfere with measurements of the cosmic microwave background. Read More »

More Articles