The production of stars (also called the star-formation rate) in a galaxy changes significantly over time. Since the late '90s, several studies have shown that the star-formation rate density in the Universe is highest roughly 10 billion years ago (a redshift of 2 or roughly 3.5 billion years after the Big Bang). At later times, the rate at which galaxies are forming stars decreases by factors of 10 and more. For example, while galaxies during that time form on average 100 stars per year, our Milky Way (and similar galaxies) forms today about 1 star per year. The star-formation rate of a galaxy is coupled to the inflow of cold gas (out of which stars are being formed) onto the galaxy as well as its interaction with inter-galactic gas and other galaxies. By tracking the star formation of populations of galaxies across cosmic time, we can understand how these galaxies formed, how fast they grow, and what their internal and external conditions are.

Currently we lack of detailed information on the star formation activity of galaxies in the high-redshift Universe. Here is where ALPINE plays a crucial role by providing us with new key measurements for many galaxies during this epoch to understand galaxy formation and evolution in its earliest phases. The far-infrared continuum emission measured by ALPINE enables us to look through dense dust to see star formation taking place in dust-obscured parts of the galaxies, which would be missed by current observations in the optical light. On the other hand, ALPINE measures the emission of singly ionized Carbon (C⁺) from these galaxies, which is produced nearby hot, young stars, hence is a direct measure of the star formation productivity. While such measurements have been performed for a handful of galaxies with limited statistical conclusions, ALPINE targets 118 galaxies in total to increase the statistical power by orders of magnitudes.

Because galaxies in the early Universe form stars at much higher rates than today's galaxies, we expect their internal properties to be significantly different. A hot topic in astrophysics is to link these two very different populations of galaxies. For example, we expect early galaxies go be more rich in gas and to (maybe) have a higher efficiencty in forming stars. On the other hand, they have lower amounts of metals (as metals build up over time), which inhibits cooling, hence results in a warmer inter-stellar medium. Finally, it has been observed that the structure of early galaxies is more irregular due to star formation happening in discrete lumps (like giant Orion Nebulae).

ALPINE enables us to understand better the internal properties of a large sample of galaxies at early times. For example, ALPINE constraines the correlation between singly ionized Carbon emission (C⁺) and star-formation rate. While in today's galaxies there is a strong correlation between these quantities, the case for metal-poor early galaxies is less clear. The measurement of the spatial extent of C⁺ emission from ALPINE suggests it to be emitted in the warm diffuse gas between stars without much correlation to star formation. ALPINE also images the location and amount of dust for such a large sample of galaxies for the first time. The spatial correlation between dust, C⁺ emission, and stars (measured from optical light) tell us about the interior conditions of the galaxies, which we can compare to observations at lower redshifts.

Star formation is directly coupled to the internal structure and properties of galaxies. For example, the light profile of galaxies traces the amount of star formation. Quiescent galaxies (meaning galaxies that have stopped their star formation) are usually of spheroidal shape, with the light concentrated in their centers. Star forming galaxies have usually a disk structure with spiral arms (such as our Milky Way) but can be significantly irregular in the early Universe. Star formation is also related to the motion of gas in and outside the galaxy. Strong star formation can trigger fast outflows of gas by stellar winds.

As the sound of an ambulance gets distorted when it passes by, the light emitted by an object becomes red/blue-shifted when the object is receding or approaching at high speeds. This is called the Doppler effect. ALMA uses the Doppler effect to measure the motion of the gas with speeds a slow as 50 km/s (31 miles/s) by computing the red/blue shift of the C⁺ emission. Since we know that this light is emitted at a wavelength of exactly 157.8μm (where Carbon is singly ionized to form C⁺), deviation from that indicates motion of the gas in which this light is emitted. With this trick, ALPINE can measure how turbulent (or ordered) the gas in these galaxies is, whether there are significant outflows of gas, and how this is related to the star formation of the galaxies. While such studies have been successfully carried out in less distant galaxies, ALPINE allows such measurement for galaxies in the early Universe.

The environment in which a galaxies lives has a significant impact on its life. For example, galaxies living in denser environments (meaning more galaxies per volume) are more prone to interact with other galaxies. Galaxy collisions can result in a compaction of gas, which will lead to a higher rate of star formation (a so-called starburst). In turn, the gas is used up very quickly (as stars are being formed out of it), leaving back a gas-poor galaxy with ceased star formation. If the gas is not replenished by inflow, the galaxy is becoming quiescent. At the same time, the galaxy changes its look: from a irregular or spiral disk galaxy to an spheroidal galaxy without spiral features. Astrophysicists think this is a favorable avenue to create the massive, quiescent galaxies in today's Universe.

ALPINE can observe even very faint (in the optical light) companion galaxies around the 118 main targets by their C⁺ emission. In addition, from the wavelength shift of the C⁺ emission line, these observations measure the relative velocity differences between the main galaxy and its companions at a resolution of roughly 50 km/s (31 miles/s). This way, we can study statistically the rate at which galaxies collide as well as in what stage they are during a collision process and compare this to the star forming properties of the galaxies.