Without a little help from Einstein, we couldn’t have made this discovery.
NASA’s Hubble Space Telescope, currently celebrating its 30th anniversary,
still churns out novel discoveries. 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. (HUDF09 AND HXDF12 TEAMS / E. SIEGEL (PROCESSING))
By peering into the distant Universe, Hubble reveals galaxies from across cosmic time.
This tiny slice of the eXtreme Deep Field illustrates an important concept: if we count the number of galaxies in this image and extrapolate how many such similar images we’d need to cover the entire sky, we can get an estimate for how many galaxies would be revealed over the entire sky to Hubble’s eyes. The actual number of galaxies is significantly larger. (NASA, ESA, H. TEPLITZ AND M. RAFELSKI (IPAC/CALTECH), A. KOEKEMOER (STSCI), R. WINDHORST (ARIZONA STATE UNIVERSITY), AND Z. LEVAY (STSCI))
However, even with phenomenally deep views, most galaxies still remain undiscovered.
Galaxies identified in the eXtreme Deep Field image can be broken up into nearby, distant, and ultra-distant components, with Hubble only revealing the galaxies it’s capable of seeing in its wavelength ranges and at its optical limits. It’s important to remember that the light we see is only the light arriving right now, after journeying through the vast expanse of space. (NASA, ESA, AND Z. LEVAY, F. SUMMERS (STSCI))
Light spreads out as distance increases, rendering the earliest galaxies too faint for most observatories.
The brightness distance relationship, and how the flux from a light source falls off as one over the distance squared. A galaxy that’s twice as far away from Earth as another will appear only one quarter as bright. (E. SIEGEL / BEYOND THE GALAXY)
Additionally, the Universe’s expansion stretches the light’s wavelength, shifting it out of the visible range.
This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. (ROB KNOP)
However, Einstein’s idea — of mass curving space — frequently lends a helping hand.
An illustration of gravitational lensing showcases how background galaxies — or any light path — is distorted by the presence of an intervening mass, but it also shows how space itself is bent and distorted by the presence of the foreground mass itself. When multiple background objects are aligned with the same foreground lens, multiple sets of multiple images can be seen by a properly-aligned observer. (NASA/ESA)
Intervening concentrations of matter between ourselves and a distant object can stretch, distort, and magnify its light.
The distant lensed galaxy, nicknamed the Sunburst Arc, has its light arriving now from when the Universe was just 3 billion years old. The lens magnifies and brightens the background galaxy to up to 30 times its normal apparent luminosity, revealing features as small as 520 light-years across. (NASA, ESA, AND E. RIVERA-THORSEN (INSTITUTE OF THEORETICAL ASTROPHYSICS OSLO, NORWAY))
This phenomenon — strong gravitational lensing — reveals objects otherwise too faint and distant to be seen.
The galaxy cluster MACS 0416 from the Hubble Frontier Fields, with the mass shown in cyan and the magnification from lensing shown in magenta. That magenta-colored area is where the lensing magnification will be maximized, as there is an area located a specific distance away from any given mass distribution, including galaxies and galaxy clusters, where the brightness enhancements will be maximized. (STSCI/NASA/CATS TEAM/R. LIVERMORE (UT AUSTIN))
A decade ago, the Herschel (infrared) and Planck (microwave) observatories
combined to identify lensed galaxy candidates. The below image, of one of the galaxy clusters identified by Planck and Herschel, showcases a 2016 follow-up with ALMA at very long wavelengths. The identified marks of A, B, and C all correspond to the same background galaxy, lensed multiple times by the foreground cluster. (N. NESVADBA ET AL. (2016), ARXIV:1610.01169)
performed with Hubble, at last revealed their details. Six of the galaxy clusters identified by Planck and Herschel were imaged here by Hubble, showcasing the ultra-distant starburst galaxy PLCK G045.1+61.1 in the lower-left panel. (BRENDA L. FRYE ET AL. (2019) APJ, 871 51)
Here, a background galaxy —
PLCK G045.1+61.1 — appears as multiple red dots, lensed by a massive foreground cluster. Seen here in incredible detail, thanks to the NASA/ESA Hubble Space Telescope, is the starburst galaxy formally known as PLCK G045.1+61.1. The galaxy appears as multiple reddish dots near the center of the image and is being gravitationally lensed by a cluster of closer galaxies that are also visible. (ESA/HUBBLE & NASA, B. FRYE)
It’s a single star-forming galaxy, appearing only 1.9 billion years after the Big Bang.
The same image, with the three images of the ultra-distant background galaxy highlighted here. These multiple images have different length light-paths to Earth; if a supernova goes off in this galaxy, we’ll observe its light arriving at three different times. (ESA/HUBBLE & NASA, B. FRYE)
The stars within are intrinsically blue; the red color arises from cosmic expansion.
Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. Galaxies from just 2 billion years after the Big Bang are dominated by stars with much bluer colors than our Milky Way, but appear red (or infrared) due to the expanding Universe. (NASA AND ESA)
Using similar techniques, NASA’s upcoming James Webb Space Telescope will shatter our earliest galaxy records.
As we’re exploring more and more of the Universe, we’re able to look farther away in space, which equates to farther back in time. The James Webb Space Telescope will take us to depths, directly, that our present-day observing facilities cannot match, with Webb’s infrared eyes revealing the ultra-distant starlight that Hubble cannot hope to see. (NASA / JWST AND HST TEAMS) Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more. Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.