The Hubble Space Telescope: Three Decades of Revelations

A window open to the cosmos

Launched on April 24, 1990, the Hubble Space Telescope has become one of the most influential instruments in modern astronomy. Orbiting at an altitude of 547 km, it escapes Earth's atmospheric turbulence, offering an angular resolution of about 0.05 arcseconds. Hubble has explored the cosmic past up to redshifts \( z > 10 \), revealing galaxies formed less than a billion years after the Big Bang.

Its WFC3 camera and COS spectrograph have captured light from objects so distant that their visible radiation is now shifted into the infrared according to Hubble's law: \( v = H_0 \, d \) where \( v \) is the recessional velocity, \( H_0 \) is the expansion constant, and \( d \) is the distance to the observed galaxy.

N.B.:
The redshift, denoted \( z \), measures the change in wavelength of a spectral line between emission and observation. It is defined by the relation: \( z = \frac{\lambda_{obs} - \lambda_{emit}}{\lambda_{emit}} \)
A positive redshift (\( z > 0 \)) corresponds to a redshift, indicating that the object is moving away from the observer. Conversely, a negative redshift (\( z < 0 \)) corresponds to a blueshift. On a large scale, the \( z \) values observed for galaxies and quasars are proportional to their distance according to Hubble's law, \( v = H_0 \, d \), a direct signature of the expansion of the universe.

Measuring the expansion of the universe

One of Hubble's major missions was the precise measurement of the universe's expansion rate. By observing Cepheids and Type Ia supernovae, Hubble enabled Adam Riess (1969–) and Brian Schmidt (1967–) to obtain a value for the Hubble constant around \( H_0 ≈ 73 \, km·s^{-1}·Mpc^{-1} \).

These results highlighted a tension between local values of \( H_0 \) and those derived from the cosmic microwave background by the Planck satellite, which gives \( H_0 ≈ 67.4 \, km·s^{-1}·Mpc^{-1} \). This disagreement, known as the "Hubble tension," suggests that our \(\Lambda CDM\) cosmological model may need revision.

Nebulae and stellar nurseries

Hubble revealed the complex structures of nebulae, such as the famous Pillars of Creation in the Eagle Nebula (M16). These images, captured in visible and infrared light, show the dynamic processes of star formation, where winds from young stars shape molecular clouds.

The details of the jets from HH objects or the protoplanetary disks of Orion have helped us understand the mechanisms of accretion and ejection of matter around protostars.

Exoplanets and distant atmospheres

Hubble also contributed to the spectroscopy of exoplanetary atmospheres. By analyzing starlight filtered through the atmospheres of planets during transits, it detected the presence of water vapor, methane, and sodium in several extrasolar worlds.

These pioneering observations paved the way for more recent instruments like the JWST, capable of studying the chemical composition of these atmospheres with greater precision.

The limits of the visible

The iconic image of the Hubble Ultra Deep Field (2004) brings together nearly 10,000 galaxies in a tiny field of 11 arcminutes. It illustrates the staggering density of the observable cosmos: each point of light represents an entire galaxy containing billions of stars.

N.B.:
The limiting magnitude reached by the Hubble Ultra Deep Field is \( m_{AB} ≈ 30 \), or objects 4 billion times fainter than those visible to the naked eye.

A lasting vision

Despite its age, Hubble remains a cornerstone of space observation. Its data, archived for over thirty years, are still used for research on dark matter, quasars, and galactic morphology. The interaction between Hubble and the James Webb Space Telescope will now provide a complementary view of the cosmos, combining ultraviolet and deep infrared.

Source: NASA HubbleSite, ESA, and ADS – Astrophysics Data System.