Even though it’s been nearly 100 years since its initial discovery, the expanding Universe still puzzles almost everyone who thinks about it. What causes the Universe to expand? Why did it start off expanding in the first place? What determines the rate of expansion, and how does that rate translate into something that we can actually go and observe? What do we mean when we say “the expansion is accelerating,” and why is that such a profound, revolutionary statement that we only started making in the 1990s? And, behind the scenes, what is the ultimate cause of each aspect of our cosmic expansion, and how confidently can we state such things? There’s a remarkable scientific story to be told here, and even seasoned scientists and science communicators frequently make mistakes when trying to provide answers to these questions. It makes educating the general public, especially young people, a particular challenge for father-of-a-curious-13-year-old, Philip Gee, who writes in to ask: “I read everything you write, absorb 5% and talk to my kids like I’m an expert haha… I so often read this sort of thing:
The Enigma of Cosmic Expansion
The concept of a universe that expands might sound strange, but it’s an idea that has been confirmed by a century of astronomical observations. When we look at galaxies in all directions, we see them moving away from us. This phenomenon is known as the redshift of light, and it’s a direct consequence of cosmic expansion.
Albert Einstein’s theory of general relativity, published in 1915, predicted that the Universe could be expanding or contracting. However, he introduced the cosmological constant, a fudge factor, to keep the Universe static, as it was believed to be at the time. When Edwin Hubble made the groundbreaking observation in 1929 that galaxies were receding from us in all directions, it became clear that the Universe was, indeed, expanding.
The Expansion’s Origin
While we know the Universe is expanding, we still grapple with the question of why it started expanding in the first place. The answer to this question remains one of the most profound mysteries in cosmology. Some theories suggest that a sudden burst of cosmic inflation in the early Universe initiated this expansion, while others propose exotic fields or particles. The true origin of cosmic expansion remains a subject of intense research and debate.
The Rate of Expansion
The rate at which the Universe expands is determined by a parameter known as the Hubble constant. This constant represents the current rate of cosmic expansion and helps us measure the age of the Universe. Scientists have refined their measurements of the Hubble constant over the years, but there is ongoing debate and uncertainty in its precise value. This discrepancy is a subject of great interest and debate within the scientific community.
The Accelerating Expansion
In the 1990s, scientists made a groundbreaking discovery that shook the foundations of cosmology: the expansion of the Universe is accelerating. This revelation was based on observations of distant supernovae and led to the idea of dark energy, a mysterious force that counteracts the attractive pull of gravity and causes the Universe to expand at an accelerating rate. The exact nature of dark energy is still not fully understood, and it remains one of the most significant unsolved problems in astrophysics.
The Ultimate Cause of Expansion
Understanding the ultimate cause of cosmic expansion remains a challenge for scientists. While we have made remarkable progress in our knowledge of the Universe, there are still many unanswered questions. The cosmic web of galaxies, dark matter, dark energy, and the interplay of fundamental forces all contribute to the complex tapestry of our expanding Universe. Scientists continue to explore and refine our understanding of these components, seeking the ultimate cause of cosmic expansion with confidence and dedication.
In conclusion, the story of cosmic expansion is a captivating journey that has intrigued humanity for nearly a century. From its discovery by Edwin Hubble to the enigma of its origins and the mysteries of accelerating expansion, this field of study has continually pushed the boundaries of our understanding of the Universe. As we navigate the uncharted territories of dark energy and the ultimate cause of expansion, scientists and science communicators work tirelessly to convey the wonders of our cosmos to the curious minds of the world, ensuring that even young enthusiasts like Philip Gee can continue to be inspired by the enigmatic beauty of our expanding Universe.
