What we’ve learned after 35 years of NASA’s Hubble

This photo shows the Hubble Space telescope being deployed, on April 25, 1990, one day after its launch. It was taken by the IMAX Cargo Bay Camera (ICBC) mounted aboard the space shuttle Discovery. Originally launched to an altitude of ~620 km, Hubble is now about ~100 kilometers lower as of May 2024, and will continue its orbital decay due to atmospheric drag.

This deep-field region of the GOODS-South field contains 18 galaxies forming stars so quickly that the number of stars inside will double in just 10 million years: just 0.1% the lifetime of the Universe. The deepest views of the Universe, as revealed by space telescopes, take us back into the early history of the Universe, where star-formation rates were much greater than today, but where fewer than 1% of the Universe’s cumulative stars had already formed. Many of the most distant galaxies are found in close proximity to other foreground galaxies, whose mass distorts and magnifies the light from background objects.

The Hubble eXtreme Deep Field (XDF) may have observed a region of sky just 1/32,000,000th of the total, but was able to uncover a whopping 5,500 galaxies within it: an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice. The remaining 90%+ of galaxies are either too faint or too red or too obscured for Hubble to reveal, but when we extrapolate over the entire observable Universe, we expect to obtain a total of upward of 2 trillion galaxies: up to 6-20 trillion, at present.

Only because the most distant galaxy spotted by Hubble, GN-z11, is located in a region where the intergalactic medium is mostly reionized, was Hubble able to reveal it to us at the present time, breaking the prior record held by EGSY8p7. Other galaxies that are at this same distance but aren’t along a serendipitously greater-than-average line of sight as far as reionization goes can only be revealed at longer wavelengths, and by observatories such as JWST. At present, GN-z11 has been relegated to the 14th most distant galaxy known as of May 2025, nearly 3 years into the JWST era.

The combination of Subaru data (red image) and Hubble data (blue image) reveals the presence of an exoplanet at a distance of 93 Astronomical Units (where 1 A.U. is the Earth-Sun distance) from its parent star. The luminosity of the massive object indicates reflected stellar emission rather than unimpeded direct emission, while the lack of a polarization signal is highly suggestive of a formation scenario other than core accretion. This is one of more than 5000 exoplanets presently known.

These data points, sorted by year, show different measurements of the expansion rate of the Universe using the cosmic distance ladder method, with the data points falling into two main groups: one clustered around 50 km/s/Mpc and one clustered around 100 km/s/Mpc. The results of the Hubble Key Project, released in 2001, are shown with the red bars.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (along with many others) cannot be sufficiently explained.

The impressively huge galaxy cluster MACS J1149.5+223, whose light took over 5 billion years to reach us, is among the largest bound structures in all the Universe. On larger scales, nearby galaxies, groups, and clusters may appear to be associated with it, but are being driven apart from this cluster due to dark energy; superclusters are only apparent structures, but the largest galaxy clusters that are bound can still reach hundreds of millions, and perhaps even a billion, light-years in extent.

This tiny sliver of the GOODS-N deep field, imaged with many observatories including Hubble, Spitzer, Chandra, XMM-Newton, Herschel, the VLT, and more, contains a seemingly unremarkable red dot. That object, a quasar-galaxy hybrid from just 730 million years after the Big Bang, showcases how bright and powerful quasars can be. Many of the “little red dots” seen by JWST and other observatories are brightness-enhanced by the activity of the central black hole, with some jets pointing directly along our line-of-sight.

The visible/near-IR photos from Hubble show a massive star, at least 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. The direct collapse of this particular object, while still under investigation, may have been triggered by a stellar companion.

In this comparison view, the Hubble data is shown in violet, while ALMA data, revealing dust and cold gas (which themselves indicate star-formation potential), is overlaid in orange. With its views out beyond the limits of infrared astronomy but sensitive to spectroscopic features, ALMA can detect some of the most distant ionized/excited elements in cosmic history.

JWST’s diffraction spikes, seen in great detail around the star 2MASS J17554042+6551277, are the same spikes seen in the first successful alignment image. The science data, as evidenced by the glorious detail of background galaxies, has helped revolutionize what we know about the Universe since it first began science operations in 2022. Complemented by Hubble and many other observatories, our cumulative knowledge about the Universe only increases.