How Space Telescopes Let Us See 13.5 Billion Years Into the Past

A Photograph That Broke Open the Universe

In December 1995, astronomers pointed the Hubble Space Telescope at a patch of sky near the Big Dipper—a region so apparently empty that some colleagues thought it was a waste of observing time. The telescope stared at that same dark patch for 10 consecutive days, collecting light. The resulting image, the Hubble Deep Field, showed roughly 3,000 galaxies in a region of sky about one-thirteenth the diameter of the full moon. Almost every smear of light in that image was an entire galaxy, containing hundreds of billions of stars. That 5.3 square arcminute patch of "empty" sky contained the universe's history for 12 billion years. In 2022, the James Webb Space Telescope surpassed that achievement, returning images of galaxies that formed when the universe was less than 400 million years old—only 3% of its current age.

How Light Becomes a Time Machine

Light travels at 299,792 kilometers per second—the cosmic speed limit. Nothing travels faster. This means that light reaching us from distant objects has been traveling for the time it took to cross the intervening space, and we see those objects as they were when the light departed, not as they are now. This is "lookback time." The Sun is 8 light-minutes away; we see it as it was 8 minutes ago. The Andromeda Galaxy is 2.537 million light-years away; we see it as it was 2.537 million years ago.

For the most distant objects JWST has detected—galaxies at redshift z ≈ 16—lookback time reaches approximately 13.5 billion years, meaning the light left those galaxies when the universe was roughly 270 million years old. The Big Bang occurred 13.8 billion years ago. JWST is observing structures that formed in the universe's first 2% of its existence.

Redshift: How Expansion Stretches Light

The universe is expanding. Space itself is growing between galaxies, and as light travels through expanding space, its wavelength is stretched—shifted toward the red end of the spectrum. The more the light is redshifted, the farther it has traveled and the longer ago it was emitted. Astronomers measure this with a dimensionless quantity called z (redshift). A redshift of z=1 means the universe has doubled in size since the light was emitted; z=10 means it has expanded 11-fold.

Why Infrared: The James Webb Advantage

Hubble sees primarily in visible and ultraviolet light. The problem is that highly redshifted light from the early universe—originally emitted as ultraviolet or visible wavelengths—has been stretched into the infrared band by the time it reaches Earth. Hubble cannot detect infrared wavelengths with sufficient sensitivity to see the first galaxies.

The James Webb Space Telescope (JWST), launched December 25, 2021, and reaching operational orbit by January 2022, was specifically designed for this problem. Its primary mirror is 6.5 meters in diameter (vs. Hubble's 2.4 meters)—a gold-coated beryllium mosaic that collects 6.25 times more light. JWST operates at wavelengths from 0.6 to 28 micrometers, covering near-infrared and mid-infrared bands. To detect faint infrared signals, its instruments must be cooled to -233°C (40 Kelvin), just above absolute zero, using a five-layer sunshield the size of a tennis court.

• JWST's primary mirror area: 25.4 square meters vs. Hubble's 4.5 square meters
• JWST operating temperature: 40 Kelvin (critical for mid-infrared detector sensitivity)
• Cost: approximately $10 billion USD after 25 years of development
• Orbit: L2 Lagrange point, 1.5 million km from Earth — too far for astronaut servicing
• Design lifetime: 10 years (with fuel reserves potentially enabling 20+ years)

Cosmic Dawn: What JWST Found in the Early Universe

Within its first year of operations (2022–2023), JWST overturned several assumptions about early galaxy formation. Astronomers expected the first galaxies to be small, irregular, and faint—slowly accumulating mass over hundreds of millions of years. Instead, JWST found massive, bright, surprisingly well-structured galaxies at z > 10, some of which contain billions of stars and display orderly disk structures that astronomers didn't expect to form so quickly after the Big Bang.

A galaxy designated JADES-GS-z14-0, confirmed in 2024 at z ≈ 14.32, dates to when the universe was approximately 290 million years old—the most distant spectroscopically confirmed galaxy as of that date. It already contains roughly 500 million solar masses of stars. How that much stellar mass assembled so quickly conflicts with standard galaxy formation models and is driving active theoretical revision.

The Cosmic Microwave Background: Seeing Even Farther Back

Space telescopes observe light from stars and galaxies. But there is an observational boundary even JWST cannot cross with light: the epoch of recombination, approximately 380,000 years after the Big Bang. Before that moment, the universe was so hot and dense that it was opaque—photons couldn't travel freely. After recombination, the universe became transparent, releasing a bath of photons that has since been redshifted to microwave wavelengths. This is the Cosmic Microwave Background (CMB), mapped in extraordinary detail by the Planck satellite (2009–2013).

Seeing the Universe's First Light

The ultimate observational frontier lies beyond even JWST's reach: the period before the first stars ignited, called the cosmic dark ages. Existing between approximately 380,000 and 200 million years after the Big Bang, this era produced no light—only cold hydrogen gas slowly clumping under gravity. Radio telescopes tuned to the 21-centimeter hydrogen emission line, redshifted to meter wavelengths, may eventually detect signatures from this period. Projects like the Square Kilometre Array (SKA) under construction in South Africa and Australia aim to map hydrogen emission from this era, extending the observational timeline almost to the Big Bang itself. JWST sees 13.5 billion years back. SKA may push that to 13.7 billion. What each new instrument reveals is not just the universe's past—it is the ongoing rewriting of our models of how everything came to exist.