Is a new anomaly affecting the entire Universe?

Sorry, astronomers: the expanding Universe doesn’t add up.

unreachable

The galaxies shown in this picture all lie beyond the Local Group, and as such are all gravitationally unbound from us. As a result, as the Universe expands, the light from them gets shifted towards longer, redder wavelengths, and these objects wind up farther away, in light-years, than the number of years it actually takes the light to journey from them to our eyes. As the expansion relentlessly continues, they’ll wind up progressively farther and farther away.

(Credit: ESO/INAF-VST/OmegaCAM. Acknowledgment: OmegaCen/Astro-WISE/Kapteyn Institute)

The largest anomaly is the Hubble tension.

expansion of the Universe

Two of the most successful methods for measuring great cosmic distances are based on either their apparent brightness (left) or their apparent angular size (right), both of which are directly observable. If we can understand the intrinsic physical properties of these objects, we can use them as either standard candles (left) or standard rulers (right) to determine how the Universe has expanded, and therefore what it’s made of, over its cosmic history. The geometry of how bright or how large an object appears is not trivial in the expanding Universe.

(Credit: NASA/JPL-Caltech)

Two expansion rate measurement methods yield incompatible values.

The cold spots (shown in blue) in the CMB are not inherently colder, but rather represent regions where there is a greater gravitational pull due to a greater density of matter, while the hot spots (in red) are only hotter because the radiation in that region lives in a shallower gravitational well. Over time, the overdense regions will be much more likely to grow into stars, galaxies, and clusters, while the underdense regions will be less likely to do so. The evidence of the imperfections in the CMB and in the large-scale structure of the Universe provide a way to reconstruct the expansion rate.

(Credit: EM Huff, SDSS-III/South Pole Telescope, Zosia Rostomian)

The early relic method, via cosmic imperfections, yields 67 km/s/Mpc.

Pantheon+

Although there are many aspects of our cosmos that all data sets agree on, the rate at which the Universe is expanding is not one of them. Based on supernovae data alone, we can infer an expansion rate of ~73 km/s/Mpc, but supernovae will not probe the first ~3 billion years of our cosmic history. If we include data from the cosmic microwave background, itself emitted very close to the Big Bang, there are irreconcilable differences at this moment in time, but only at the <10% level!

(Credit: D. Brout et al./Pantheon+, ApJ submitted, 2022)

The distance ladder method, from individually measured objects, yields 73 km/s/Mpc.

Measuring back in time and distance (to the left of “today”) can inform how the Universe will evolve and accelerate/decelerate far into the future. By linking the expansion rate to the matter-and-energy contents of the Universe and measuring the expansion rate, we can come up with a value for a Hubble time in the Universe, but that value isn’t a constant; it evolves as the Universe expands and time flows on.

(Credit: Saul Perlmutter/UC Berkeley)

But another cosmic imperfection anomaly is similarly puzzling.

expansion of the Universe

Using the cosmic distance ladder means stitching together different cosmic scales, where one always worries about uncertainties where the different “rungs” of the ladder connect. As shown here, we are now down to as few as three “rungs” on that ladder, and the full set of measurements agree with one another spectacularly.

(Credit: AG Riess et al., ApJ, 2022)

Consider the cosmic microwave background (CMB): leftover radiation from the Big Bang.

According to the original observations of Penzias and Wilson, the galactic plane emitted some astrophysical sources of radiation (center), but above and below, all that remained was a near-perfect, uniform background of radiation. The temperature and spectrum of this radiation has now been measured, and the agreement with the Big Bang’s predictions are extraordinary. If we could see microwave light with our eyes, the entire night sky would look like the green oval shown.

(Credit: NASA/WMAP Science Team)

Although mostly uniform, one direction is ~3.3 millikelvin hotter while the opposite is similarly cooler.

Although the cosmic microwave background is the same rough temperature in all directions, there are 1-part-in-800 deviations in one particular direction: consistent with this being our motion through the Universe. At 1-part-in-800 the overall magnitude of the CMB’s amplitude itself, this corresponds to a motion of about 1-part-in-800 the speed of light, or ~368 km/s from the perspective of the Sun.

(Credit: J. Delabrouille et al., A&A, 2013)

This “CMB dipole” reflects our Sun’s relative motion to the CMB: of ~370 km/s.

An accurate model of how the planets orbit the Sun, which then moves through the galaxy in a different direction-of-motion. The distance of each planet from the Sun determines the amount of overall radiation and energy that it receives, but this is not the only factor at play in determining a planet’s temperature. Additionally, the Sun moves through the Milky Way, which moves through the Local Group, which moves through the larger Universe.

(Credit: Rhys Taylor)

Our Local Group moves much faster: ~620 km/s.

This illustrated map of our local supercluster, the Virgo supercluster, spans more than 100 million light-years and contains our Local Group, which has the Milky Way, Andromeda, Triangulum, and about ~60 smaller galaxies. The overdense regions gravitationally attract us, while the regions of below-average density effectively repel us relative to the average cosmic attraction.

(Credit: Andrew Z. Colvin/Wikimedia Commons)

This should be due to cosmic, gravitational imperfections tugging on us.

Because matter is distributed roughly uniformly throughout the Universe, it isn’t just the overdense regions that gravitationally influence our motions, but the underdense regions as well. A feature known as the dipole repeller, illustrated here, was discovered only recently and may explain our Local Group’s peculiar motion relative to the other objects in the Universe.

(Credit: Y. Hoffman et al., Nature Astronomy, 2017)

Nearby galaxy motions consistently support this picture.

The motions of nearby galaxies and galaxy clusters (as shown by the ‘lines’ along which their velocities flow) are mapped out with the nearby mass field. The greatest overdensities (in red/yellow) and underdensities (in black/blue) came about from very small gravitational differences in the early Universe. In the vicinity of the most overdense regions, individual galaxies can move with peculiar velocities of many thousands of kilometers per second, but what is seen is consistent, overall, with our observed local motion through the Universe.

(Credit: HM Courtois et al., Astronomical Journal, 2013)

However, more distant motion tracers conflict with it.

On scales larger than our local supercluster, or more than a few hundred-million light-years, we no longer see differences in various directions that correspond to our expected, measured motion through the Universe. Instead, the observed effects are inconsistent, both with the local Universe’s measurements and with each other in many cases.

(Credit: Andrew Z. Colvin and Zeryphex/Astronom5109; Wikimedia Commons)

Plasmas within clusters indicate smaller overall motions: below ~260 km/s.

The Planck satellite’s measurements of the CMB temperature on small angular scales can reveal enhancements or suppressions of temperature by tens of microkelvin induced by the motions of objects: the kinetic Sunyaev-Zel’dovich effect. From galaxy clusters, they see an effect consistent with 0, and that’s substantially weaker than one would expect from our inferred motion through the Universe.

(Credit: Websky Simulations)

The brightest cluster galaxies, however, reveal larger motions: ~689 km/s.

largest galaxy

The giant galaxy cluster, Abell 2029, houses galaxy IC 1101 at its core. At 5.5-to-6.0 million light-years across, over 100 trillion stars and the mass of nearly a quadrillion suns, it’s the largest known galaxy of all by many metrics. A survey of the brightest galaxy within all of the Abell clusters reveals a cosmic motion that’s inconsistent with the CMB dipole.

(Credit: Digitized Sky Survey 2; NASA)

X-ray emissions reveal giant ones (in the wrong direction!) of ~900 km/s.

The anisotropies in X-ray galaxy cluster counts are much greater in magnitude and also in the wrong direction than expected from our motion through the Universe: another example of a surprising but important cosmological tension.

(Credit: K. Migkas et al., A&A, 2021)

And anisotropies in galaxy counts reveal more than double the expected effect.

All-sky maps of galaxies reveal that there are more galaxies found at the same brightness/distance thresholds in one direction over another. This so-called Rocket Effect has a predicted amplitude from the dipole seen in the CMB, but what’s observed is more than double the predicted effect.

(Credit: T. Jarrett (IPAC/Caltech))

Radio galaxy counts are even worse: four times the expected amplitude.

When the entire sky is viewed in a variety of wavelengths, certain sources corresponding to distant objects beyond our galaxy are revealed. In radio wavelengths, galaxies can be seen in all directions, but the slight difference in one set of directions over another appears significantly greater than the difference that would be expected from our observed motion through the Universe.

(Credit: ESA, HFI and LFI consortia; CO map from T. Dame et al., 2001)

Quasar counts from WISE possess the same problem.

With its all-sky infrared survey, NASA’s Wide-field Infrared Survey Explorer, or WISE, has identified millions of quasar candidates, identified all across the sky (and shown in a small region here) with yellow circles. The clustering of quasars shows an anomalously large signal in terms of one direction having higher quasar counts (and the opposite having lower counts) than expected by a far greater amount than our observed motions lead us to expect.

(Credit: NASA/JPL-Caltech/UCLA)

Larger-scale, upcoming surveys could robustly confirm this second “Hubble tension.”

The European Space Agency’s EUCLID mission, scheduled for launch in 2023, will be one of three major endeavors this decade, along with the NSF’s Vera Rubin observatory and NASA’s Nancy Roman mission, to map the large-scale Universe to extraordinary breadth and accuracy.

(Credit: European Space Agency)

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