Exploring the Magnification of Distant Galaxies in an Expanding Universe
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Chapter 1: The Paradox of Distant Galaxies
When we observe distant galaxies, we instinctively recognize that their size can be misleading. A three-dimensional perspective is essential to accurately gauge their scale, especially in an expanding universe where distances are constantly changing. This intriguing phenomenon raises a question posed by Patreon supporter Ken Blackman:
Do ancient galaxies seem larger to us than their actual size due to the universe's expansion? And if so, to what extent?
Initially, distant objects appear smaller, but at a certain point, they may actually seem larger. This counterintuitive truth is rooted in the concept of angular diameter, which behaves differently in the context of cosmic expansion.
For instance, consider the playful activity of holding two fingers near your eyes while looking at a person nearby. The illusion of "squishing" their head works because of angular size, a concept that applies universally, not just on Earth.
The angular size of an object relies on its actual size and its distance from the observer. As light spreads, the energy received diminishes with the square of the distance, illustrating that larger objects, viewed from afar, can take up the same angular size in the sky.
Let's delve deeper into this concept.
Section 1.1: Understanding Angular Size
You may assume that the perceived size of an object is simply a function of its physical size and distance. For example, if we move the Moon—currently about 380,000 km away—farther away, it would appear significantly smaller. This reasoning holds in a static universe, but our reality is quite different.
The universe is dynamic and expanding. Early on, radiation dominated, leading to a decrease in energy density as both volume and wavelength increased. Eventually, matter began to dominate, and around 6 billion years ago, dark energy emerged, influencing how we perceive distances and sizes in the universe.
Subsection 1.1.1: The Role of Dark Energy
As we look into the universe, we notice two primary methods for measuring vast cosmic distances: based on apparent brightness and apparent angular size. Understanding the intrinsic properties of these cosmic entities allows us to calculate the universe's expansion and its composition through time.
When observing an object made up of two light sources on an invisible rod, the angle perceived is directly related to its distance in a flat universe. However, in an expanding universe, we must account for the evolving shapes and sizes of cosmic entities.
Chapter 2: The Counterintuitive Reality of Cosmic Distances
The surprising reality is that in our dark energy-dominated universe, distant galaxies may initially appear smaller, but past a certain distance, they can seem larger again. For instance, if the Milky Way were placed at the distance of the Andromeda galaxy (about 2.5 million light-years), it would occupy more than 2 degrees of the sky, shrinking down to a minimum size of 3.6 arc-seconds at approximately 14.6 billion light-years away.
As we explore deeper into the cosmos, we observe that different galaxies exist at various distances and redshifts, providing insight into the universe's expansion and its historical changes.
Section 2.1: The Angular Scale in Cosmic Observations
The universe consistently presents the same number of square degrees in the sky, yet the physical size corresponding to angular scales varies with distance. A small angular scale is one arc-second, representing the distance from Earth to the Sun if viewed from one parsec away.
When examining cosmic distances, we primarily measure redshift, which indicates how much the spectral lines of atoms and ions shift. As we venture further, we see that more parsecs fit into one arc-second, with this maximum occurring at a redshift of approximately 1.5, or around 14.6 billion light-years.
This unique phenomenon, where distant objects seem larger than expected, is crucial for astronomers. High-resolution images captured from observatories can reveal extraordinary details about galaxies that are billions of light-years away.
In summary, to accurately assess how large an object appears in our expanding universe, we must comprehend both its physical size and the dynamics of universal expansion. The universe contains approximately 68% dark energy, 27% dark matter, 5% normal matter, and 0.01% radiation, indicating that while distant objects may seem smaller, they can also be magnified due to the universe's expansion.
The most distant galaxy observed, GN-z11, appears twice the size of a similarly sized galaxy that is only half its distance. This surprising effect arises solely from the universe's expansion, independent of gravitational lensing.
Feel free to send your questions to Ask Ethan at [email protected]!
Starts With A Bang can also be found on Forbes and is republished on Medium with a 7-day delay. Ethan has authored two books: Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.