雅思阅读 48: Ripples in Spacetime（时空中的涟漪）

改编由 Caltech / LIGO / ScienceAlert（2025年8-12月）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://thisis.caltech.edu/news/possible-superkilonova-exploded-not-once-but-twice

Reading Passage

A. When gravitational-wave detectors first turned on in 2015, they were looking for a needle in a haystack. The Laser Interferometer Gravitational-Wave Observatory, or LIGO, operates twin instruments in Louisiana and Washington State, each using laser interferometers that measure changes in the length of their four-kilometre vacuum tubes equivalent to a fraction of the diameter of a proton. The first signal, labelled GW150914, arrived from two colliding black holes about 1.3 billion light-years away, confirming a prediction Einstein had made a century earlier. A decade later, that needle-in-a-haystack exercise has become routine: by March 2025, the international LIGO–Virgo–KAGRA network had logged its two-hundredth gravitational-wave candidate in the current observing run, known as O4. The detectors have moved from discovery to census-building, mapping a population of merging black holes and neutron stars across a substantial slice of the observable universe, and refining estimates of how often such collisions occur. A third detector, KAGRA in Japan, joined the network in 2020, and a fourth, VIRGO in Italy, has been operating since 2017, allowing the exact position of each event to be triangulated in the sky.

B. The signal that arrived on 18 August 2025 was, however, not routine. Designated S250818k, it was a "sub-threshold" candidate — weaker than the network's formal detection threshold, meaning it might have been noise — but its shape pointed toward a binary neutron-star merger at a distance of roughly 237 million parsecs, or about 770 million light-years. What made it strange was its "chirp mass", a quantity derived from the rate at which the two objects spiral inward as they lose energy to gravitational radiation. The inferred mass was surprisingly low: low enough that at least one of the neutron stars appeared to weigh less than the Sun. Neutron stars below one solar mass are not predicted by standard stellar-evolution theory, because a progenitor star massive enough to collapse into a neutron star should leave behind a compact remnant heavier than that. The few known sub-solar compact objects have usually been explained as white dwarfs — the inert cores of low-mass stars — not as neutron stars. The alternative, suggested by some theorists, is that one of the objects is a black hole so light that it could only have formed in the early universe, before stars existed — a so-called primordial black hole.

C. Within minutes of the alert, optical telescopes across the world began scanning the patch of sky indicated by the gravitational-wave triangulation. The Zwicky Transient Facility, a wide-field camera at Palomar Observatory in California, identified a rapidly fading point of light inside that region — an object named AT2025ulz. At first glance it resembled a kilonova: the radioactive fireball that lights up when two neutron stars merge and spray heavy elements such as gold and platinum into space. But the light curve behaved oddly. It did not follow the smooth, monotonically fading decay expected from a standard kilonova, powered as such objects are by a single radioactive ejecta cloud. Instead, it appeared to brighten, fade, and then brighten again — as though a second, separate explosion had gone off inside the first, weeks after the initial collision. Researchers at Caltech, who announced the finding in December 2025, proposed that the object might be a "superkilonova": a neutron-star merger so unusual that it detonated not once but twice. The second peak, they suggested, could mark the moment when the collapsed core crossed some internal threshold and rebounded.

D. The interpretation is still contested. A sub-threshold signal, by definition, sits close to the noise floor, and random fluctuations can mimic real astrophysical events. The optical transient's double-peaked light curve could also be explained by dust obscuration that temporarily blocks part of the fading ejecta, by interaction with surrounding gas that re-energises the emission weeks later, or by a completely different class of explosion that merely coincided with the gravitational-wave trigger in time. Until at least two independent lines of evidence converge, astronomers will continue to argue about what they are actually seeing. Radio follow-up observations, reported in 2026, have detected a faint radio counterpart at the same position, which strengthens the case for a physical connection, but does not by itself prove the superkilonova model. The Pan-STARRS survey, which imaged nearly a third of the localisation region within days, found no other obvious candidate, which also favours the association. Until a second, similarly unusual event is recorded — with a confident gravitational-wave detection this time — AT2025ulz will remain a tantalising anomaly rather than a confirmed new phenomenon. Astronomers have been here before: a handful of apparent "first detections" in the 1990s turned out to be instrumental noise once the detectors were upgraded.

E. Why does it matter? Gravitational-wave astronomy was invented to test general relativity and to detect collisions that emit almost no light. It has already confirmed that black holes merge with the masses Einstein's equations predict, and that neutron-star collisions forge the heavy elements found in planetary systems, including the gold in jewellery. A sub-solar neutron star, if real, would force astrophysicists to rewrite the stellar-evolution textbooks; a double explosion would reveal a new endpoint in the life of compact objects, perhaps involving a phase transition inside nuclear matter. Beyond the physics, the event illustrates how the field has matured. A single 2015 signal was a decade-long project of analysis and confirmation. A 2025 signal was triangulated within minutes, followed optically within hours, modelled within months, and debated in the literature within a year. The era of gravitational-wave astronomy, once a quiet ripple in a Louisiana laboratory, is now a global enterprise — and the next anomaly may already be on its way. The proposed next-generation detectors, with ten times the sensitivity of O4, could turn such anomalies into routine data points within a decade.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. An unusually light signal ii. The optical counterpart and a strange double burst iii. Why the interpretation remains uncertain iv. Why the discovery matters and how the field has matured v. A brief history of general relativity vi. How LIGO's lasers are manufactured vii. A list of all two-hundred O4 events

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. What was GW150914? A. The first gravitational-wave detection, from merging black holes. B. A neutron star inside the Solar System. C. A lunar impact. D. A laboratory laser.

6. Why was S250818k unusual? A. It was detected in visible light first. B. Its inferred chirp mass suggested at least one sub-solar-mass neutron star. C. It came from inside the Milky Way. D. It was the loudest signal ever recorded.

7. What optical transient was identified in the signal's direction? A. AT2025ulz. B. GW150914. C. O4. D. KAGRA.

8. Why is the superkilonova interpretation still contested? A. No optical transient was ever found. B. The sub-threshold signal sits near the noise floor and other explanations remain possible. C. The event was too far away to observe. D. LIGO was offline at the time.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. By March 2025, the LIGO–Virgo–KAGRA network had logged its two-hundredth O4 candidate.
10. S250818k was located about 237 million light-years from Earth.
11. A neutron star below one solar mass is predicted by standard stellar-evolution theory.
12. Radio follow-up observations detected a counterpart at the same position as AT2025ulz.
13. KAGRA, the Japanese detector, is located on Mount Fuji.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

The optical counterpart AT2025ulz showed a double-peaked light curve, leading Caltech researchers to propose it might be a "(14) __________" — a merger that detonated not once but (15) __________.

答案与解析

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