The vast majority of galaxies across the cosmos are moving away from our own Milky Way, a phenomenon known as galactic recession. This observation confirms that the universe is not static but dynamic. This systemic retreat of galaxies reveals a uniform pattern that provides insight into the history of the cosmos.
How Scientists Measure Galactic Movement
Scientists determine the speed and direction of galaxies by analyzing the light they emit using the Doppler effect. This physical principle describes how the frequency of a wave changes relative to a moving observer, causing a shift in the observed color of light.
When a galaxy is moving away from Earth, the light waves it emits are stretched out, causing their wavelengths to appear longer. Since the red end of the electromagnetic spectrum corresponds to longer wavelengths, this stretching is known as redshift. The greater the observed shift toward the red end, the faster the galaxy is receding from us.
Conversely, if a galaxy is moving toward us, the light waves are compressed, causing their wavelengths to appear shorter. This results in a shift toward the blue end of the spectrum, which is called blueshift. By measuring the precise amount of redshift or blueshift in the characteristic spectral lines of elements like hydrogen and helium, astronomers can calculate a galaxy’s exact velocity toward or away from our location.
The Universal Relationship Between Distance and Speed
The systematic nature of galactic recession was first quantified by astronomer Edwin Hubble in the late 1920s. He observed a specific mathematical pattern: the speed at which a galaxy moves away from us is directly proportional to its distance from us. This means that a galaxy twice as far away appears to be receding at roughly twice the speed.
This linear relationship is often summarized by the equation \(v = H_0D\), where \(v\) is the galaxy’s recessional velocity, \(D\) is its distance, and \(H_0\) is the Hubble constant. Hubble’s original work provided the first strong observational evidence for an expanding universe. The current value of the Hubble constant is estimated to be approximately 70 kilometers per second per megaparsec.
This observation of an increasing velocity with increasing distance suggests that the universe is not static. If galaxies were simply flying apart from a single central explosion, the relationship between distance and speed would not be so uniform. Instead, the observed pattern implies a universal, uniform expansion that affects the space between all distant objects equally.
Understanding the Expansion of Space
The observed pattern of galactic recession is best explained by understanding that galaxies are not moving through space, but that the space between them is stretching and expanding. This intrinsic expansion of the cosmos affects the fabric of spacetime everywhere, increasing the separation between gravitationally unbound structures.
A common way to visualize this concept is to imagine a loaf of raisin bread dough rising in an oven. As the dough expands, every raisin moves farther away from every other raisin. A raisin that is far away will have more dough stretching between it and a reference raisin, causing it to move away at a greater apparent speed. Crucially, the raisins themselves do not move within the dough; the dough carries them apart.
Similarly, in the cosmos, the galaxies are like the raisins, and space is the dough. This analogy helps to illustrate why there is no single center to the expansion; an observer in any galaxy would see all other distant galaxies receding from them according to the same rule. Furthermore, this expansion does not cause galaxies, stars, or even atoms to grow larger because the local forces holding these structures together are overwhelmingly stronger than the force of cosmic expansion.
The light from distant galaxies is also affected by this stretching space. As the photons travel across billions of light-years, the space they occupy expands, which directly lengthens their wavelength. This is the cosmological redshift, and it serves as a direct measurement of how much the universe has grown since the light began its journey.
Gravitational Exceptions to Universal Recession
While the expansion of space governs motion on the largest scales, gravity remains the dominant force on smaller, localized scales. This means that not every galaxy is moving away from us, as strong gravitational pull can overcome the effects of cosmic expansion.
Our own galactic neighborhood, known as the Local Group, is a prime example of this exception. This group includes the Milky Way, the Andromeda galaxy, the Triangulum galaxy, and dozens of smaller dwarf galaxies, all bound together by mutual gravitational attraction. Within this group, the galaxies are not receding.
The most notable exception is the Andromeda galaxy, which is currently blueshifted, meaning it is moving toward the Milky Way at a speed of about 110 kilometers per second. This motion is caused by the immense gravitational forces between the two massive galaxies, which are pulling them toward a future collision. On these local scales, the expansion of space is negligible compared to the power of gravity.