Assessing the potential of LIGO-India in resolving the Hubble tension

This paper demonstrates through end-to-end simulations that adding LIGO-India to the global detector network will significantly accelerate the resolution of the Hubble tension by boosting gravitational wave detections, improving sky localization, and increasing kilonova follow-up rates, potentially reducing the time required to precisely measure the Hubble constant from decades to just a few years.

The Gist

The Big Problem: The Universe's Speedometer is Broken

Imagine the universe is a giant car speeding away from us. Astronomers want to know exactly how fast it is going (this speed is called the Hubble Constant).

The problem is that the car's speedometer has two different readings:

1. The "Early" Reading: Looking at the baby universe (the Cosmic Microwave Background) suggests the car is going a certain speed.
2. The "Late" Reading: Looking at the adult universe (supernovas) suggests it is going faster.

These two numbers don't match. This disagreement is called the "Hubble Tension." If the difference isn't just a mistake in the math, it might mean our understanding of physics is incomplete. To fix this, scientists need a third, independent way to measure the speed to see which one is right.

The Solution: Gravitational Waves as "Standard Sirens"

The paper proposes using Gravitational Waves (GW)—ripples in space-time caused by massive objects crashing together—as a new speedometer.

• The Analogy: Imagine two black holes or neutron stars colliding like two tuning forks hitting each other. They make a "sound" (a gravitational wave) that tells us exactly how loud the crash was (the distance).
• The Missing Piece: To calculate the speed of the universe, you need two things: Distance (which the wave gives you) and Speed (which comes from the Redshift of the galaxy where the crash happened).
• The Catch: The gravitational wave tells you how far away the crash was, but it doesn't tell you which galaxy it happened in. It's like hearing a car crash in the distance but not knowing which street it was on. You need to look at the sky with a telescope to find the specific galaxy and measure its speed.

The Current Bottleneck: The "Flashlight in a Storm" Problem

Currently, we have three main "ears" listening for these crashes: LIGO (in Washington), LIGO (in Louisiana), and Virgo (in Italy).

• The Localization Issue: With only three ears, it's hard to pinpoint exactly where the sound came from. The "search area" on the sky is huge—like a flashlight beam that is so wide it covers an entire city.
• The Telescope Problem: To find the galaxy, telescopes have to scan that huge city. They have to take thousands of photos of different neighborhoods (tiles) to find the crash.
• The Time Problem: The light from these crashes (called kilonovae) fades very quickly, like a firework. By the time the telescopes scan the whole city, the firework is gone.
• The Result: It would take decades to find enough of these events to fix the Hubble Tension with the current setup.

The Hero: LIGO-India

This paper argues that adding a fourth ear, LIGO-India, changes everything.

1. Tripling the "Triangulation" Power

• The Analogy: Imagine trying to find a lost hiker in a forest. With two people shouting, you get a rough idea of the direction. With three, you get a better idea. With four people spread out across a continent (USA, Europe, India), you can pinpoint the hiker's location almost instantly.
• The Paper's Claim: Adding LIGO-India shrinks the "search area" on the sky by a factor of 5. Instead of searching a whole city, the telescope only needs to search a single neighborhood.

2. The "Triple Coincidence" Boost

• The Analogy: Imagine you need three people to agree that they heard a noise to confirm it's real. If one person is on a break, you can't confirm it.
• The Paper's Claim: With four detectors, the chance that at least three are listening at the same time doubles. This means we catch twice as many events that are precise enough to be useful.

3. The "Deep Dive" Advantage

• The Analogy: Because the search area is now so small (a single neighborhood instead of a whole city), the telescope doesn't have to rush. It can spend all its time staring at that one spot, taking many photos to see very faint, distant objects.
• The Paper's Claim: This allows telescopes to see much deeper into space and catch events that would have been too faint to see before.

The Results: From Decades to Years

The authors ran computer simulations to see what happens when LIGO-India joins the party.

• More Detections: They found that LIGO-India could increase the number of useful detections by 70% just by listening better.
• Better Follow-ups: Because the location is so precise, telescopes can find the "kilonova" (the light from the crash) 2 to 7 times more often than before.
• Solving the Mystery:
  • Without LIGO-India: It might take decades to get enough data to solve the Hubble Tension.
  • With LIGO-India: The paper estimates we could solve it in just a few years (potentially by 2035 if LIGO-India starts on schedule).

Summary

The paper concludes that LIGO-India acts like a high-precision GPS for the universe. By turning a blurry, wide-angle search into a sharp, focused target, it allows telescopes to catch the fleeting light of cosmic crashes much faster. This acceleration could resolve one of the biggest mysteries in physics within a human lifetime rather than a generation.

Technical Summary

Technical Summary: Assessing the Potential of LIGO-India in Resolving the Hubble Tension

Problem Statement
The precise determination of the Hubble constant ($H_0$) remains a central challenge in cosmology due to the persistent "Hubble Tension"—a discrepancy between early-universe measurements (e.g., Cosmic Microwave Background) yielding lower values and late-universe observations (e.g., supernovae) yielding higher values. Gravitational waves (GWs) from binary neutron star (BNS) mergers offer an independent "standard siren" method to measure $H_0$. However, this method requires the joint detection of GWs and electromagnetic (EM) counterparts (kilonovae) to determine the host galaxy's redshift. Current estimates suggest that achieving a 2% precision in $H_0$ requires tens of such events. With the existing three-detector network (HLV: LIGO Hanford, LIGO Livingston, Virgo), the low detection rate and poor sky localization imply that resolving this tension could take decades. This paper assesses whether the addition of the LIGO-India detector (LIGO-Aundha) can significantly accelerate this timeline.

Methodology
The authors employ an end-to-end simulation framework to model future observation scenarios involving a four-detector network (HLVA: Hanford, Livingston, Virgo, and LIGO-India) compared against the current HLV network.

1. Data Generation:

   • A population of $\sim767$ low-spin BNS sources was generated with isotropic sky distribution, uniform in comoving volume, and redshifts between 0.005 and 0.2.
   • Neutron star masses followed a Gaussian distribution ($\mu=1.33 M_\odot, \sigma=0.09 M_\odot$), and spins were limited to $0-0.05$.
   • Signals were injected into Gaussian colored noise realizations based on projected A+ (LIGO) and Advanced Virgo Plus (AdV+) sensitivity curves.

2. GW Detection and Localization:

   • Search Pipeline: The PyCBC Live low-latency search pipeline was modified to handle a four-detector configuration. Events were identified using matched filtering with a network signal-to-noise ratio (SNR) threshold of $\hat{\rho}_c \ge 12$ for three-detector coincidences.
   • Localization: Sky localization posteriors were generated using BAYESTAR. The study analyzed how the addition of LIGO-India affects the 90% credible sky area.

3. Electromagnetic Follow-up:

   • The gwemopt toolkit was used to schedule EM follow-up observations for detected GW events.
   • Three telescopes were modeled: Zwicky Transient Facility (ZTF), Vera C. Rubin Observatory (LSST), and Wide-field Infrared Transient Explorer (WINTER).
   • Kilonova light curves were simulated using the gwemlightcurves package (based on the Dietrich et al. prescription) to determine apparent magnitudes.
   • Detection criteria required the source's apparent magnitude to be brighter than the telescope's limiting magnitude within the tiled sky area.

4. Scenarios:

   • Simulations were run for various detector duty cycles (100%, 70%, and 50%) to account for realistic operational constraints.
   • Two localization strategies were evaluated: summing probabilities over all tiles versus focusing only on the tile with the highest probability.

Key Contributions and Results

• Detection Rate Increase:

  • The inclusion of LIGO-India increases the horizon distance and, more critically, the "triple-coincidence" duty cycle (the probability that at least three detectors are operational simultaneously).
  • At a 70% duty cycle, the number of detectable BNS events increases by a factor of $\sim2.8$ compared to the HLV network. At 50% duty cycle, this factor rises to $\sim4.5$.
  • Analytical estimates suggest a $\sim70\%$ increase in the overall detection rate and a doubling of properly localized events.

• Sky Localization and EM Follow-up:

  • LIGO-India improves sky localization precision by a factor of $\sim5$, reducing the 90% credible area from hundreds of square degrees to $\sim9$ square degrees (comparable to the LSST field of view).
  • This reduction allows telescopes to spend significantly more time on fewer tiles, enabling deeper observations.
  • EM Follow-up Rates: The study finds that LIGO-India increases the rate of successfully localized kilonovae by a factor of $\sim2-7$ depending on the telescope and duty cycle.
    • For LSST at a 70% duty cycle, the probability of detecting a source increases from $\sim6.6\%$ (HLV) to $\sim20.9\%$ (HLVA) when considering all tiles.
    • When focusing on the highest-probability tile, the improvement factor is even more pronounced (up to $\sim5\times$ at 70% duty cycle).

• Hubble Constant Precision:

  • The authors calculated the fractional uncertainty in $H_0$ ($\sigma_{H_0}/H_0$) based on the improved localization and detection rates.
  • For LSST follow-ups, the uncertainty reduction factor (improvement) ranges from 1.65 (at 100% duty cycle) to 2.82 (at 50% duty cycle) when LIGO-India is included.
  • This implies that the time required to reach a 2% precision in $H_0$ is reduced from decades to a few years.

Significance and Claims
The paper claims that LIGO-India is a critical component for resolving the Hubble Tension within the next decade. By significantly boosting the detection rate of triple-coincidence events and drastically improving sky localization, LIGO-India enables deeper and more efficient EM follow-up campaigns.

• Timeline Reduction: The authors estimate that without LIGO-India, achieving the necessary precision for $H_0$ could take several decades. With LIGO-India, this timeline could be reduced to approximately 10 years (assuming current sensitivity) and potentially as short as 5 years if detector sensitivities improve to A$\sharp$ and Virgo nEXT levels.
• Modesty and Caveats: The authors explicitly note that their estimates rely on assumptions regarding the rate of BNS mergers with detectable EM counterparts (assumed to be $\sim1$ per year). They acknowledge that if the actual rate of kilonovae is lower, the timeline may extend, potentially requiring third-generation detectors (Einstein Telescope, Cosmic Explorer) in the 2040s. Conversely, if rates are higher or sensitivities improve faster, the resolution could occur by $\sim2035$.
• Conclusion: The study concludes that the combined advantages of LIGO-India—specifically the increased duty cycle for coincident detections and the geometric improvement in localization—will likely reduce the observation time required to resolve the Hubble Tension from decades to a few years, making it a pivotal instrument for future cosmology.