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The universe just got even more confusing. Last week, the biggest ever 3D map of our galaxy听was released as part of the second batch of data from the European Space Agency鈥檚 Gaia satellite. The long-awaited data dump revealed the location and brightness of some 1.7 billion stars in the Milky Way.

Now the first analysis of the data has crystallised our confusion about the rate at which the universe is expanding.

We have two ways to determine the speed of the universe鈥檚 inflation, and they have always returned different values. Some researchers hoped that the data released on April 25 from the Gaia spacecraft might lessen the conflict, but they鈥檝e only made it worse.

One of our determinations of this so-called Hubble constant comes from the cosmic microwave background (CMB), a relic of the first light in the cosmos after the big bang. Researchers have used the Planck space observatory to examine this light and figure out how fast the universe was expanding back then. Those values can then be plugged into models of how the cosmos has evolved to predict how fast it should be expanding today.

The other method involves directly measuring the distances to stars called Cepheid variables, to figure out how quickly objects in the local universe are moving away from us. This more direct method has come up with a value more than 9 per cent higher than the CMB method.

Cosmic confusion

In the past, we鈥檝e only been able to measure a few Cepheids at a time, but Gaia pinpointed 50 of them. at the Space Telescope Science Institute in Baltimore, Maryland and his colleagues analysed Gaia鈥檚 Cepheid data to see how it would affect the Hubble constant discrepancy.

鈥淣ot only is it confirmed, but it鈥檚 actually reinforced,鈥� Riess says. Prior to this analysis, he says, the听probability of this听apparent discrepancy arising in the data by chance was 1 in 1000 鈥� now, that has fallen to only 1 in 7000.

If the discrepancy is real, it means that something is wrong with our models of the universe鈥檚 evolution. And it鈥檚 looking more and more real.

Reiss says there may be more particles out there that we鈥檝e never detected, or maybe our guesses about the natures of dark matter and dark energy are wrong.

鈥淲hen we say the Hubble constant should be lower, that鈥檚 with models using the most vanilla, least interesting versions of dark matter and dark energy,鈥� says Riess. 鈥淏ut maybe there鈥檚 a wrinkle. Maybe it鈥檚 much weirder.鈥�

Galactic scars

Closer to home, the Gaia data have also revealed a disturbance in the Milky Way. Our galaxy doesn鈥檛 float alone in space; it is surrounded by smaller satellite galaxies. These are gravitationally bound to our own galaxy, so astronomers agree it is likely that some of them interacted with the Milky Way in the past, perhaps by smashing through the galaxy鈥檚 disc.

Thanks to Gaia, we now have evidence that another galaxy perturbed the Milky Way鈥檚 disc relatively recently. at the University of Barcelona in Spain and her colleagues analysed the motions of more than 6 million stars from the Gaia dataset, and found patterns we鈥檝e never seen before.

Plots of these stars鈥� velocities have swoops, arches and spirals indicating patches of stars that are moving together. If the Milky Way were in equilibrium, and hadn鈥檛 been recently perturbed, those patterns wouldn鈥檛 appear. That they do indicates that something has shaken up the stars recently enough that their orbits haven鈥檛 relaxed back to a stable state yet.

The researchers鈥� analysis found that the Milky Way was likely perturbed between 300 and 900 million years ago, which corresponds to the last time the Sagittarius dwarf galaxy is estimated to have made a close pass.

鈥淭his is just the beginning for Gaia,鈥� says Riess. 鈥淕aia should be delivering data that鈥檚 five or six times more precise than this in a few years.鈥� And even for now, analysis of the deluge of data we just received is nowhere near complete.

Reference:听

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