Galactic Redshift and Expansion

What is galactic red shift?

Galactic redshift is a concept in astrophysics that pertains to the observation that the light from distant galaxies is shifted towards the red end of the spectrum. This phenomenon is intimately tied to the expansion of the universe. To understand galactic redshift, we'll need to cover a few key concepts: the electromagnetic spectrum, the Doppler effect, and the expansion of the universe.

1. Electromagnetic Spectrum: Light, along with radio waves, X-rays, and others, is part of the electromagnetic spectrum. This spectrum is like a rainbow of different types of radiation, distinguished by wavelength. Red light has a longer wavelength than blue light.

2. Doppler Effect: This is a phenomenon you experience when an ambulance passes you with its siren on. As it approaches, the sound is higher-pitched, and as it moves away, the pitch lowers. This happens because the sound waves are compressed as the source approaches and stretched as it moves away. Light behaves similarly. When a light source moves away from us, its light appears redshifted (stretched to longer wavelengths).

3. Expansion of the Universe: Edwin Hubble discovered that galaxies are moving away from us and that the farther away a galaxy is, the faster it seems to be moving. This is due to the expansion of the universe. As the universe expands, it stretches the light traveling through it, shifting it towards the red end of the spectrum.

An analogy to help understand this is to think of the universe like a loaf of raisin bread baking in the oven. As the bread rises, it expands, and the raisins (galaxies) move further apart. If you imagine light as a stretchy ribbon connecting the raisins, as the bread expands, the ribbon (light) stretches, corresponding to a redshift.

To test your understanding, can you explain how the Doppler effect relates to the redshift of galaxies?

Light waves are shifted toward longer wavelengths, I.e., the red portion of the spectrum.

Excellent! You've correctly understood the key point: light waves from galaxies moving away from us are shifted toward longer wavelengths, which corresponds to the red portion of the electromagnetic spectrum. This is analogous to the Doppler effect, where the frequency of a wave changes due to the motion of the source relative to the observer.

Now, let's delve a bit deeper into the concept of the expansion of the universe, which is closely linked to galactic redshift.

The universe is expanding, meaning the space between galaxies is stretching. This isn't like objects moving through space but rather space itself expanding. Imagine dots on a balloon. As the balloon inflates, the dots move away from each other. In the context of the universe, these dots represent galaxies. As space expands, the light traveling through it also stretches, leading to redshift. This stretching doesn't affect the speed of light but changes its wavelength and frequency.

To solidify your understanding, here's a question: If you were observing a galaxy that was moving toward us instead of away, what kind of shift would you expect to see in its light, and why?

I would observe a blueshift, since the light waves would be compressed, I.e., the wavelength would become shorter.

That's absolutely correct! If a galaxy were moving towards us, we would observe a blueshift. This is because the light waves would be compressed, leading to shorter wavelengths, which is analogous to the higher-pitched sound of an ambulance siren as it moves towards you. This compression results in the light shifting towards the blue end of the electromagnetic spectrum.

Having grasped the concepts of redshift and blueshift as they relate to the motion of galaxies, let's explore the implications of galactic redshift on our understanding of the universe:

1. Cosmological Implications: The observation of galactic redshifts supports the theory that the universe is expanding. This expansion is a key aspect of the Big Bang theory, which describes the universe's origin as a rapid expansion from a hot, dense state. As we observe more distant galaxies, their light is more redshifted, indicating they are moving away faster, which is consistent with an expanding universe.

2. Hubble's Law: Formulated by Edwin Hubble, this law states that the recessional velocity of a galaxy (how fast it's moving away) is proportional to its distance from us. This discovery was crucial in shifting the scientific consensus from a static universe to one that is expanding.

Now, for a more conceptual question to test your understanding: How does the observation of galactic redshift support the Big Bang theory?

According to the Big Bang theory, the universe is expanding from its dense origin, so galaxies are moving away from one another. An observer would see a redshift as a result of this movement.

Exactly right! The Big Bang theory posits that the universe began in a hot, dense state and has been expanding ever since. This expansion causes galaxies to move away from each other. An observer on Earth, or anywhere else in the universe, would see most galaxies moving away, evidenced by the redshift in their light. The greater the redshift, the faster a galaxy is moving away, which aligns with the universe's expansion from its origin.

This understanding helps in comprehending the dynamic nature of the universe and its evolution over billions of years. It's a cornerstone concept in cosmology, shaping our view of the universe's past, present, and future.

Let's deepen your understanding with another question: If the universe is expanding, what does this imply about the future of the universe? How might this expansion affect the galaxies and the cosmic structures over a very long period of time?

At a certain point the speed of the expansion must become limited, since nothing can move faster than the speed of light.

Your response touches on a critical aspect of cosmic expansion, but it needs a bit of clarification. The expansion of the universe is a unique process where the speed of light as a limit applies differently than it does for objects moving through space.

In the context of the universe's expansion, it's not the galaxies themselves moving through space at speeds approaching or exceeding the speed of light. Rather, it's the space itself that is expanding. This expansion can, theoretically, occur at rates that cause galaxies to recede from each other at speeds greater than the speed of light. This doesn't violate the laws of physics because it's not the galaxies moving through space at superluminal speeds, but the space between galaxies expanding.

Now, regarding the future of the universe, this continuous expansion has several implications:

1. Increasing Isolation of Galaxies: Over vast periods, galaxies will move further apart, leading to increasing isolation. Eventually, galaxies in our local group might be so far from others that they're no longer visible, with the observable universe being limited to our local group.

2. Dark Energy and Accelerating Expansion: The discovery that the universe's expansion is accelerating, attributed to dark energy, suggests a future where galaxies move apart faster and faster. This could lead to a scenario called the "Big Freeze" or "Heat Death," where the universe becomes so vast and cold as stars burn out and new ones cease to form.

3. Eventual Fate: The exact fate of the universe depends on factors like the nature of dark energy and the total density of matter in the universe. Other theories, like the Big Crunch (where the universe eventually collapses in on itself) or the Big Rip (where the expansion accelerates to the point of tearing apart all matter), provide alternative scenarios.

Given these points, here's a related question: How does the concept of dark energy relate to the accelerating expansion of the universe?

It is thought to directly contribute to the accelerating expansion of the universe. — My question: is the concept of dark energy universally accepted (pun intended) or is it controversial? Are there competing concepts?

The concept of dark energy is widely accepted in the scientific community, but it's important to note that it's more of a placeholder for something we don't fully understand yet. The term "dark energy" refers to an unknown form of energy that is hypothesized to permeate all of space and to be responsible for the observed acceleration of the expansion of the universe.

1. Current Understanding and Acceptance: The discovery that the expansion of the universe is accelerating, which was made in the late 1990s, was unexpected. This discovery led to the hypothesis of dark energy as the driving force behind this acceleration. The concept is broadly accepted because it fits well with the observed data, particularly the measurements of distant supernovae, the cosmic microwave background radiation, and the distribution of galaxies.

2. Controversial Aspects: While the existence of dark energy is generally accepted, its nature is one of the biggest mysteries in physics. There are different theoretical models for what dark energy could be. The simplest explanation is that it is a "cosmological constant," a constant energy density filling space homogeneously. Other theories suggest it could be a dynamic field changing over time.

3. Competing Concepts: There are alternative theories to dark energy. Some scientists propose modifications to general relativity (the theory of gravity by Albert Einstein) that could account for the accelerated expansion without needing dark energy. These alternative theories, however, often face challenges in matching the precision of data that the dark energy model explains.

Given this context, the concept of dark energy, while widely accepted, is still an area of active research and debate. There's a lot we don't know, and as our observational technologies improve and we gather more data, our understanding might evolve.

To assess your grasp of this concept, could you explain why the discovery of the universe's accelerating expansion was surprising to astronomers?

If one thinks of the Big Bang as a form of explosion, the speed of the matter moving away should remain constant or maybe even diminish if there is some sort of friction. The observed acceleration would indeed be unexpected.

You've grasped a key point: the expectation that the expansion of the universe might slow down over time, much like how an explosion's initial thrust weakens. This expectation was based on the understanding of gravity as an attractive force. The reasoning was that the gravitational attraction among all the matter in the universe would gradually slow the expansion initiated by the Big Bang. This slowing down could be likened to throwing a ball upwards on Earth, where gravity gradually decelerates its ascent.

The surprising discovery of the universe's accelerating expansion challenged this view. Instead of observing a slowing down due to gravitational pull, astronomers found that galaxies are moving away from each other at an increasingly faster rate. This observation implied the presence of a repulsive force or energy, countering the effects of gravity on cosmic scales. This is where the concept of dark energy comes into play, acting like a mysterious force that is pushing galaxies apart, contrary to the expected pull of gravity.

This discovery was so unexpected and groundbreaking that it led to the 2011 Nobel Prize in Physics being awarded to the scientists who made this observation.

To further explore this concept, consider this question: How might the continuous acceleration of the universe's expansion affect the long-term future of the cosmos? What are the implications for the fate of galaxies, stars, and the overall structure of the universe?

The force pushing galaxies and stars further and further apart would lead to the death of the universe, as star and galaxy formation would cease.

Your understanding aligns well with one of the proposed scenarios for the long-term future of the universe, often referred to as the "Big Freeze" or "Heat Death." This scenario, influenced by the concept of dark energy and the accelerating expansion of the universe, suggests a gradual and inevitable decline into a cold, dark state. Here's a breakdown of this process:

1. Increasing Isolation of Galaxies: As the universe expands, galaxies will become increasingly isolated. The accelerating expansion means that, eventually, galaxies outside of our local group (the galaxy cluster that includes the Milky Way) will recede beyond the observable universe. They'll move away so fast that their light will never reach us.

2. Star Formation and Death: Over trillions of years, the supply of gas needed for new star formation will be exhausted. Existing stars will burn out, leaving behind white dwarfs, neutron stars, and black holes.

3. Dark Era of the Universe: As even the longest-lived stars die, the universe will enter a dark era. Only the faint glow of white dwarfs and other stellar remnants will remain, gradually fading as they release their residual heat.

4. Degradation of Matter: On even longer timescales, it's hypothesized that all matter, including black holes, might eventually decay due to processes like Hawking radiation, leaving a universe filled with low-energy photons and other fundamental particles.

This scenario assumes that dark energy continues to drive the universe's expansion at an accelerating rate and that the nature of dark energy doesn't change over time. However, it's important to note that our understanding of dark energy and the long-term fate of the universe is still evolving, and new discoveries could lead to revisions of these predictions.

Given this understanding of the universe's potential fate, how does the concept of the Big Freeze differ from other end-of-the-universe scenarios, such as the Big Crunch or the Big Rip?

In the Big Rip, the continued acceleration is thought to eventually “tear apart” space and matter. I’m not sure what exactly would lead to a Big Crunch - why would the universe collapse in on itself? It would need to hit a proverbial wall - what would serve as such a wall ?

You've correctly described the Big Rip scenario. Now, let's clarify the Big Crunch and its distinction from the Big Rip and the Big Freeze:

1. Big Crunch: The Big Crunch is a hypothetical scenario where the expansion of the universe eventually reverses, leading to a contraction. This would occur if the density of the universe is high enough that gravity overcomes the expansion driven by the Big Bang. Instead of a "proverbial wall," it's the gravitational attraction of all the matter in the universe pulling everything back together. In this scenario, the universe would collapse in on itself, potentially leading to a state akin to the one before the Big Bang. This could be envisioned as the reverse process of the Big Bang, where everything in the universe converges to a singular point.

2. Big Rip: As you noted, in the Big Rip scenario, the expansion of the universe accelerates to such an extent that it eventually tears apart all structures in the cosmos, from galaxy clusters down to atoms themselves. This would be the result of an increasingly dominant and repulsive dark energy, leading to a catastrophic end to the universe.

3. Big Freeze/Heat Death: This scenario, which we discussed earlier, involves the universe continuing to expand at an accelerating rate, leading to a cold, dark, and empty universe. Star formation ceases, existing stars burn out, and the universe gradually moves towards a state of no thermodynamic free energy.

The Big Crunch theory has lost some favor in recent years due to observations suggesting the universe's expansion is accelerating, not slowing down. However, it's important to remember that our understanding of cosmic phenomena is constantly evolving with new data and theories. The exact fate of the universe remains one of the great mysteries in cosmology.

Given these various scenarios, how do you think our current understanding of dark energy impacts our predictions about the universe's fate?

We have introduced the concept of dark energy to explain the accelerating expansion of the universe. But our understanding is limited, so to me this looks like a crutch, or, as you said before, some sort of placeholder until we gather additional information and refine our theories.

Your characterization of dark energy as a "crutch" or placeholder in our current understanding is quite astute. It reflects a common perspective in the scientific community about the nature of dark energy and its role in cosmological models. Here's a more nuanced look at this:

1. Dark Energy as a Placeholder: Indeed, dark energy is a term used to describe whatever it is that's causing the observed acceleration of the universe's expansion. We don't yet understand what dark energy is; it's a label for an observed effect whose cause is unknown. It fits into Einstein's theory of general relativity as a form of energy with a strong negative pressure.

2. Impact on Theories and Models: The introduction of dark energy into cosmological models allows scientists to make predictions and observations that align well with the data we have, particularly regarding the universe's expansion rate and the distribution of galaxies. However, because we don't fully understand what dark energy is, there's a significant gap in our knowledge. This gap leaves room for new theories and discoveries that could profoundly alter our understanding of the universe.

3. Continuous Search for Understanding: The scientific community is actively researching to understand dark energy better. This includes observational projects like mapping the distribution of galaxies and measuring the rate of expansion of the universe at different times in its history. There's also theoretical work exploring the nature of dark energy, whether it's a constant (like the cosmological constant in Einstein's equations) or something dynamic that changes over time.

4. Implications for the Universe's Fate: Our limited understanding of dark energy means that predictions about the long-term future of the universe are tentative. If dark energy remains constant, the Big Freeze scenario seems likely. If it changes over time, other scenarios might become more probable.

To deepen your understanding, think about this: What kinds of astronomical observations or technological advancements might help us gain a better understanding of dark energy and, consequently, the fate of the universe?